The video presents the same old naive garbage of the *key* obstacles to vibrato being freedom of motion and joint flexibility in the left hand/fingers.  You can easily discredit this theory simply by producing the said conditions without the bow and then as soon as you try to integrate it with the bow, the hand locks up.  How come?  The freedom of motion and flexibility is there so why doesn't it automatically work without difficulty? 

Beth is not instructing, she is simply presenting a final product, continuous vibrato, which anyways is too advanced for the email complaints she receives.  She presents no pathway to vibrato, in terms of stages of development.  No list of dependencies or notated exercises.  Its all very trivial and of course it simply doesn't work that way.

I am starting to get  the impression its also has to do with building the muscles to get the wrist moving correctly. As its not like anything else I do at all. 

soma5, tension (along with flexibility) is just another undefined word.  The solution to tension, is another undefined word 'relax'.  Its like taking a placebo for a medical condition.

I also think a lot of people who believe they have a functional vibrato in fact have an irregular vibrato whereby the elements (amplitude and speed) do not exhibit regularity.  This is seen most vividly (vibrato stops completely for a split second) seen at the change of bow.

Practicing longer is another cure-all that I find unacceptable.  After all, we adults just can't economically justify full-time study of the violin.  I'm already attending the University of Hard Knocks for Violin and my patience is wearing thin.

Unfortunately, Miss Beth does not address the core issue:  independent control of two cyclical movements of differing frequencies and amplitudes.  Or as they say in Layman, its a juggling act.  99% of the people who watch this video will misdiagnose the problem as screeeech does:  "building the muscles to get the wrist moving correctly".  Or to put it another way, a little motor must exist in the hand.  To me, this just ends up increasing the mystical tension which makes the problem worse.

Personally, my take on vibrato is that its a problem of activating the correct *groups* of nerves throughout the entire arm.  If you try to search for the source of the motor in vibrato, you just end up increasing tension by activating the wrong nerves (which is how I define tension) However, obviously, its impossible to consciously activate the right nerves (as if we knew which ones in the first place which we dont) and as I said before relaxing adds nothing to the picture, so what to do?

Well, commonsense must rule.  For starters, a good amount of dexterity is required for a juggling act.  For the violin, a correct static hold of the instrument is not enough.  Short shifting (eg a semitone scale on one string) is obviously an act that requires considerably more dexterity in handling the instrument.  This post is already too long so I will stop here.