This is the standard method on classical guitar (which is fretted, I believe 
I certainly canāt, but Justin does mention in his video that he can tell the difference between a bent string vibrato (which only oscillates down in pitch) and āstandardā vibrato, which (typically) only oscillates up.
(I think my standard vibrato oscillates around the the in-tune note, i.e. both up and down. It needs plenty of work, in any case.)
And isnāt that what Justin meant in the video that you didnāt watch Brian that I postedā¦or do you mean something else?
I love this discussion. Hereās how I think about it.
By fretting the string, weāre effectively shortening its length, and by bending it (in any direction), weāre increasing the tension on it, both of which increase the frequency/pitch of the note being played. Vibrato essentially increases the tension/pitch briefly, returns to the initial fretted note, and repeats however many times you bounce it. I see no way to drop below the fretted note, because youād have to decrease the tension or lengthen the string, and the fret provides the lower limit there (as @brianlarsen has said).
As Justin says, I think the āclassicalā vibrato technique has some component of tightening/loosening the string tension by the friction of your finger pulling/pushing it along its length, but I also think thereās a significant component of @brianlarsenās Hard-pressed frets at work here. Itās more difficult to keep the string pinned against the fret as you move your finger away from it, so you will naturally apply more force; that, in turn will stretch the string downward toward the fretboard. Itās the difference between these two diagrams:
and
By oscillating between these two positions, weāre combining tightening/loosening the string directly along its length and ābendingā it toward the fretboard then releasing it. I strongly suspect the latter is the greater effect. Weāve all heard the change in pitch when we fret too hard or too far away from the fret.
Whatever effect we can get from āpullingā the string toward the bridge with this method is the only mechanism I can see for bring the pitch below the fretted note.
Also remember not all frets are made equally.
Classic guitar and a lot of early guitars had very low and narrow frets.
Making this less an issue.
With nylon strings you can increase tension that way because they have very little tension to start with.
Modern electric or acoustic guitars tend to have a lot more fret, it helps lead/bending in general
Jim,
The way you āwas doing itā immediately brought B.B. King to mindā¦ maybe not his exact technique but close!
My thinking 
ā¦
If it works for B.B. , it works!!!
My 2 cents!
Tod
Apologies, my friend, but that relates to classical guitar with low tension, nylon strings.
Iām afraid I āSantaClausedā my beloved signature classical to my daughter and it now lives in Germany 
so I have no way of testing hypothesesā¦ (Would you like me to tell you the Tommy Emmanuel story again? 
)
Iām lost (but itās already way too late to think), but are you talking about acoustic vibrato all the time?
and 
That was such a long time ago and there have been many possibilities in the meantimeā¦so it would have been nice to do it againā¦sometimes
But Iām really confused now and Iām just too tired to read it again, Iāll do it tomorrowā¦ 
I mean from 3;15 in the video is methode to and from the bridge and nut ?
Ok, so I did try it and yes, I can hear some vibrato coming out. However, itās nowhere near the amount of vibrato you get when doing one of the conventional vibrato methods.
And it makes sense if you think about it. How much are you elongating the string when you push it down against the fretboard? About the height of the fret wire, which is only a fraction of the amount (typically) when you vibrate the string from side-to-side.
I also did a little test: I tried to measure how much the pitch of a string increases when pushing it down hard. Using my tuner, I fretted the D string at fret 5 giving me a G note. Using a very light touch, I tuned the string. Then I pushed down very hard (careful not to move the string sideways) on the string and the tuner says it increased either 0 or 1 unit (it was not too stable). On my tuner thatās at most one tenth of a semi-tone or 10 cents. A 10 cent difference in pitch is sort of at the edge of human perception (according to my 10 minutes of research on Wikipedia), but how many cents we can perceive varies from person to person and depends on the pitch itself, the timbre, etc. One interesting thing I noticed: the string goes more out of tune if I push down far away from the fret wire. I could consistently get 10-20 cents out-of-tune with sloppy fretting technique (this was true even if not pressing down hard).
Anyway, the point is, thatās a very small shift in pitch. Iām guessing that when people perceive a shift in pitch when pressing down too hard, they are actually imparting some sideways pressure on the string, which contributes to the increase in pitch.
Anyway, my little test makes me think that @brianlarsen 's in-out vibrato (I would like to trademark this name) is also caused by accidental sideways movement of the string.
Sweet (but not too sweet) dreams
If you really want to see a master at vibrato, find a video/film of Paul Kosloff (Freeā¦early 70ās). There is an old tale, regarding Free, as the opening act for Blind Faith. While touring in the States. during a sound check at the Forum in Los Angeles, Clapton was sitting back in the shadows, in a floor seat, listening to Freeās sound check and was blown away by Kosloffās sound and vibrato technique, later to take Kosloff aside to learn the technique.
Evidently, Kosloff had learned and developed the technique while taking instruction for cello, when he was a young lad.
A mountain of Marshallās and a 59 Burst.
Just had a quick look on the Google-beast
Bit of trivia:
Violin vibrato modulates between lower pitch and the played note (by lengthening the distance from finger to bridge) as opposed to guitar which raises the pitch by increasing tension, which means it is impossible to play violin style vibrato on a guitar 
Interesting. It seems on a violin you get vibrato by changing the exact position where the string is pressed against the fingerboard and hence the length of the vibrating string. This is not possible on guitar, because the fret position defines the length of the vibrating string.
Classical vibrato on guitar is achieved by increasing (and decreasing?) the tension on the string by pulling it (and pushing it?) along the direction of the string.
They look similar when performed, since the finger moves back and forth along the string direction. But the physics is different. Interesting!
Aha, So also impossible on an acoustic because it also has frets and so Justin explains it incorrectly here at 2:55
Do you now understand my confusion after these words of yours and he even does it on the electric one, but by moving that finger in that way he does nothing other than press the strings harder and softer on the fretā¦ Was THAT so difficult to explain to me before sleeping?
I have X-rated ant-dreamsā¦ 
Paul Kossoff
You can play the violin style of vibrato on the guitar but the effectiveness will be limited. At a minimum it can add some warble or sustain. You can sharpen the note a bit (for better or worse) depending on how hard you push down.
I think that itās safe to say that over the years, guitarists have employed every possible method to vary the pitch of a note. In fact the āsoundā of many of our favorite guitar heroes is partially defined by how they use vibrato. It probably makes sense to just experiment with all the different methods, figure out what works best for you in the style of music you play and then use that.
Iām just splitting hairs here, Clint, but with violin vibrato, thereās no added pressure. You just move your hand gently away towards the nut, flattening the distal finger joint, resulting in a slightly longer string, which lowers (flattens) the tone.
On a guitar you can only increase tension, sharpening the note, as you say.
Useless information, but interesting
@Rider2040 I think itās safe to say, no matter how I wiggle my finger/string, my vibrato is likely to suck for a good while longer
A Vibrato - confusingly called a tremolo by Fender, while Fender incorrectly calls the tremolo on its amplifiers vibrato - (Italian for āvibratingā) or whammy bar is a type of lever mounted on the bridge or tailpiece of an electric guitar that ensures the tension of the strings becomes higher or lower
ā¦ 
Weāre talking fingers on a fretboard here, mijnheer
You could also take a pair of pliers with one hand, clamp the string and pull it towards the nut, which would also lower the toneā¦
