Is there a theory behind what makes it work?
F=ma -- Force = mass * acceleration -- aka Newton's law of inertia.
When you apply a force, you apply an acceleration, not a speed. If you want to put something at speed v, it starts slower and you keep pushing it until you reached desired speed. The heavier the object, the smaller the acceleration will be for an equal force. Since our limbs (and robots motor) have limited ability in the force they can produce, the "slow in/slow out" effect is increased on a hammer vs. a 2-cent coin.
When you stop applying a force (i.e. perfectly compensate for gravity), the object keeps its current velocity and direction.
