its not directional vs explosives, if youre in green cover youll take half damage from grenades no matter where the grenade lands.
if you fire a riflegrenade from the rear flank or side flank of a rifle squad thats in green cover thats bunched up=theyll get wiped in one hit from full health as if they werent in cover or in yellow cover
If you fire it at a full health rifle squad frontally thats in green cover then the squad will not get wiped at all,theyll lose a model or two and a lot of health,but they wont get wiped unless they were wounded or low on men already.
If you fire it at a bunched squad in red cover. they all die.
If you fire it at a bunched squad in yellow cover,they all die
if you fire it at a bunched squad thats only partially in green cover, and still bunched they'll probably die,depends on how the nade was aimed