The head piece projects the outlines of the court on to the wearer's environment, wherever that happens to be at the time. If the wearer is not standing on a deep, wide horizontal surface, then their interpretation of the distorted lines is all part of the challenge of this form of the game. The head
piece also remembers the player's position relative to this virtual court at the point when the player last struck the ball. It ignores the player's absolute position.
The wiiish arm piece is self-explanatory.
The mailing process might take more than one form. I've considered text messages to mobile phones, but also RFID'd widgets delivered by snail-mail. In any case, the mailed payload alerts the receiving player, and gives them quite a short notice period before the court-projection switches on and the un-paused, projected ball goes zooming past them.
The killer shots in this game are the ones timed so that the opponent receives them while inside a shower cubicle, or walking across a busy road.