Even numbers behind 4 stations. Start with quite a tight 5m square.
Start with one ball with everyone passing to the next player in sequence, e.g. (A to B), (B to C), etc.
The pass must be received half-way between the two stations. This involves throwing slightly ahead of the player, so they can catch without breaking stride.
Players move to station they passed to.
After players settle into a rhythm:-
Reverse the flow of play (without stopping, simply call 'Reverse!')
Pause briefly and introduce a second ball - this requires some synchronicity so balls don't get clumped
Stop after a couple of minutes, increase the square size to 8m and restart with both balls.
