Balancer
The balancer is fitted to control pulsing within the engine. Without any form of balancer, the engine would pulse each time the crankshaft rotated. This pulsing would be felt as a vibration which would amplify as the engine speed was increased.
The balancer has the effect of a pair of counterbalance weights which create an equal amount of energy in the opposite direction, and at the same time as that produced by the crankshaft, pistons and connecting rods. Because the opposing pulses occur at the same point of crankshaft rotation, and are of an equal magnitude, a state of equilibrium or balance is reached.