Motion could be done in few ways,
just look at the things in logical sense and you'll figure it out.
Can you move low E upwards?
No you can't, and that means that the only option is downwards, all good.
What about A?
Well, there is some space for pushing upwards but safe bet is to go downwards only, yeah.
And what about D?
D can be bended upwards without any problems, even up to tone and a half.
There's no need for that wide vibrato on D string most of the time tho, just saying.
So with D string you go both up and down.
Same with G string.
And when you do it from wrist you can actually do both up and down movement
with ONE wrist move, that's pretty economical btw.
Let me explain this for you.
Let's say you play note D on G string, 7th fret and you start applying
vibrato from neutral and you go up, that's one motion for your wrist.
And now you're going back, crossing neutral spot and continuing down, all with one wrist move.
Up only, down only, up and down, doesn't matter as long as it sounds good, try them all!