People who have never been attacked seriously tend to play as if marking strikes joint torque or the pressure of a dummy knife is not really there in practice and worse they tense up and resist tension causing themselves to be controlled and causing injury.
Take on long nail and introduce it to the group. Have everyone press the tip to their soft flesh and consider if they want to resist the movement or not. (Have the group close their eyes and give one of them the nail with announcing that it will be a part of the drill) Have them work of taking each other down with just their fists placed on the body and limbs and allow them to feel under control and slow movement how it feels to be surprised by this tiny piece of reality. Repeat the drill until humility is in their bodies and there is less danger for them to harm themselves.
Simple, Effective, swift in effect