Relaxing increases chances of ghosting iirc which would increase destruction.

Ankle position at the time of contact is a major factor and is unique to the situation, there's no real substance to saying "one is better than the other".
It's entirely situational.
All I can suggest is trying different combinations on the lead up to the hit.
You need to make sure you hit a "join cluster" if you're going for a multiple dm hit (boom, in the traditional sense).
Ankle position is mainly about the area of impact, but your whole body needs to be behind a hit in order for it to be powerful enough to break multiple joints.
For punches, it's a bit harder imo because there's always the potential of your wrist just breaking on impact.
Larfen's good at punches, may be worth catching him.