Abstract: Apparatus for applying additional momentum to the movement of a body adapted to reciprocate or flex through a substantially linear or arcuate path, notably of increasing the impact velocity of a linearly travelling weight upon an object. The apparatus includes an element for retracting the body from its rest position, notably for retracting a weight from the point of impact between the weight and an object located at the rest position of the weight and an element for biasing the body towards its rest position, notably for urging the weight towards the object so as to impart additional impact velocity to the weight as it travels toward the object. The apparatus is characterized in that: a) the element for biasing the body towards its rest position is an elastic polymeric material which is retained under tension or compression when the body is in its rest position; and b) the biasing element is one which undergoes strain crystallization. Also a method for breaking or penetrating a surface using the apparatus.