Blue 1, 2 and 3 are the attackers and are trying to get into a gap between the attackers to release a shot on goal.
The defenders must move laterally and communicate with one another to block off the attack.
Each attacking threesome has 20 seconds in which they must apply pressure to the defence. After this time they must join the back of the line and the next group of attackers can attack.
Start off with man-to-man defending, so player 1 marks player 1 before then progressing into zonal defending.
For zonal defending to be effective players will have to communicate when they are passing on an attacker to their team mate and shift to the left and right as a unit to prevent gaps opening up in the defence.
Zonal defence is generally believed to be a better defence than man marking (except when the opposition has one stand out player) but is also harder for players to learn and implement.