Backhand against underspin wrist action

[pedro.dantas]

Just saw this video where basically there is no wrist action :

Is this due to the timing? (doing the stroke with the ball at max height)

Using the wrist works better at early descending phase?

 

[pedro.dantas]

But Zhang Jike did it at early descending phase without wrist as well, when should i use the wrist?

 

[blahness]

Both are using wrist, but ZJK's stroke structure is much better than his student's, you can see how stable it is and the acceleration from the dip of the ball. This is because he uses his left hip and rotates the upper body - the student is using the old outdated method of doing down to up with the hinging at the hips so the upper body is bending forwards and then unbending which imo is a short stroke.