By any means only 5 ZEDs are allowed to be knocked down at time (this rule, however, implies 5 separate instances of knockdown - each ZED can be knocked down individually only if in the world there are less than 5 already knocked ZEDs, otherwise it is possible to knockdown much bigger groups of ZEDs, for example with the single explosion). Knockdown is a special move where ZED goes into 'ragdoll' state.
