Fragmentation grenades can be thrown at opponents in cover to force them to keep their heads down during a charge. The lethal storm of shrapnel from exploding frag grenades will drive opponents further into cover for a few precious moments while the attackers close in.
Troops armed with frag grenades always fight simultaneously against troops in cover. Initiative values are not used, both sides roll to hit and wound at the same time before making armour saves and removing casualties.