This is a fun 2-on-2 drill that works on tipping and digging. Mark Barnard, Australian national team coach and Oregon State associate head coach, explains how it works here.
The main purpose of this drill is developing the ability to tip accurately, but it also allows blockers to work on getting off the net quickly so they're ready to make a full hitting approach. Plus, it's great for practicing communication between players.
One other benefit: Everybody gets lots of touches.