Fa jing is not hard, it is soft. The effect is hard, not the means itself.

No more than 4 ounces of pressure is applied and the moment of delivery is a split-second.
You do not have time to tense the hand.
The hand (and body) must close by itself on impact, then instantly re-open again.
Conscious strength ruins any chance you have at using fa jing, so just relax.
Be patient.

When you deliver fa jing into a focus mitt or target pad it will pass through the body and into the ground. 
It may cause the floorboards to resonate.