You are not alone on this
Most just focus on the Fh and the TS Bh to a lesser extent. I thought my slice
and volley needed no help and was amazed at how much they improved with
modern technique. Volley is just the middle portion of the slice imo.
It is pretty much the same idea though, as it is still about using the hand to
align with the ball, leading with the handle.... from above for the slice
then near contact, starting to work down and across the shot line
, opposed to
classic, which has you attempt to extend thru contact along the target line.
As the hand works across the shot line, centrifugal force and to a greater extent,
the hand position brings the racket face to bear for contact.
The hand dragging the racket is key, as you can't just work across the ball, and you
need the alignment phase
of the swing to make the across part work right!
There will often be a side aspect to the underspin and just like with TS has, you
can have a lot or a little spin on the shot depending on your intent.
We do get sideways and often can used closed or neutral for slices as well,
leading with the hitting shoulder on the Bh slice,
and it can help to move thru the slice, using momentum to adjust power for the
shot. When approaching net, I mostly use a modern slice if my approach forces
me to continue strongly thru the shot, but if contact point is higher and I can
pause, I will likely use TS for the approach to net.