How do I allow head/torso to go through the floor for an animation?

So in my game, I am trying to make a thing where if you hit a key, your character goes into the floor. I have the animation for my character to go into the ground, but when I try to use the animation in game, it stops when my torso hits the floor. I have looked around and seen that I can use collision groups, and move the Head/Torso into a collision group for the duration of the animation but I don't know how to do that. Can anyone help me solve this problem?