This for me, is the Holy Grail of volleyball shots. It's rare compared to all the other types of shots you can take (some games you will never even get an attempt to capture a shot of it). It comes without warning. It is impossible to time manually. It often requires that your camera track focus. Frequently, it will require that you adjust your camera angle while you are shooting. It is however, one of the most rewarding shots to capture.
Typically you will be shooting in portrait mode. If the defender all of a sudden makes a horizontal dive you need to do one of two things. Either zoom out enough to keep them in the frame (your plan is to crop a portrait style image into a landscape one in post, so you will lose a lot of pixels - however, it is much easier I find to keep the focus point on the subject), or rotate your camera from portrait to landscape. You will likely need to zoom out slightly here too. Most players are 6"-12" shorter than their height when they are just digging a ball, because they have bend knees and hunched shoulders. When they go airborn expect them to be stretch out with their arms in front of them, so they could easily 'grow' by 2' or more compared to their dig position.
This is the one type of shot where I find the motor drive to actually be useful. The ball could be coming at literally any speed, and visual clues from the player's expressions are inexact. So just mash down on the shutter release and hope.