This exercise is the perfect example of training for Forum Theatre. It is about using all one's strength and still not winning. During a Forum session, an actor must neither give way to the intervening spect-actor, not overwhelm him, but help him apply his strength.
Divide into pairs. Imagine a line is drawn between each pair. Pairs put their hands against one another's shoulders and begin to push. To push your opponent and cross over the line into their territory would be to win the game - in this game, you do not want to win. Give in to your partner's strength; support one another's weight, sometimes pushing harder, sometimes allowing your partner to push you harder.
Variation: Back to back (from sitting to standing)
Variation: Shoulder to shoulder
Variation: Seesaw