What is the concussive rule in Warhammer 40k and how does it impact gameplay?

In Warhammer 40k, the concussive rule affects units that are hit by concussive weapons. When a unit is hit by a concussive weapon, it reduces their Initiative stat by 1 until the end of the turn. This can impact gameplay by making affected units less effective in close combat situations, as they may strike later in the fight sequence.