Increased effectiveness is produced by gravity and heaviness in the striking limb, rather than pushing harder or striking faster.

Spontaneity is the key to fa jing. 

A tense body - with habitual holding patterns - will not provide an adequate conduit for kinetic energy to pass through.
If you can feel your own body moving, you are too tense. You should feel only the movement itself.

A free body is capable of striking instantly and spontaneously.

Fa jing looks fast but it is not a matter of speed. It is loose and sudden, not fast.
By relaxing the muscles and joints fully, the body is capable of spontaneous movement.
This is what makes fa jing seem fast - no preparation, no tensing-up beforehand.
Instant energy release.