This play begins with a pass from either P or 3.
The set will be to the Middle Hitter (2).
In this sequence the hitters are selling the fact that they are going to receive the set by starting the first step of their approach.
This is when most blockers are looking to see where the hitter is and will cause a blocker to make a decision to stay put to block.
But immediately the two hitters (1 and H) move to cover.