It's much more effective if they are grouped together waiting to catch the grenade. It also reveals the position of the soldiers in the trench.
Grenades aren't very effective in trenches, unless the victim is close by. The narrow shape and curves of the trench can shield a person from a nearby blast.
Drinking from empty cups. The way you hold, drink and move with a cup with hot liquid in it is very different from an empty cup. Whenever I see it it is all I can think about.