The operation of a trunnion bascule bridge is modeled after a balanced horizontal seesaw. On the playground seesaw, the pivot point (trunnion) is in the middle so the weights on either side of the pivot point are equal. For a bridge, the pivot point is moved to one side and the seesaw is rebalanced. The animation below shows the rebalancing of the seesaw.
Once the seesaw is balanced, a relatively small amount of energy is needed to raise / lower the bridge. For example, two approx 110 hp electric motors are used to raise one side of the Michigan Avenue bridge (approx. 4,300 tons). For comparison, a 2014 all electric Fiat 500e weighs 1.5 tons and is powered by a 111 hp motor.