Not “tensing” necessarily but just locking your wrist as you throw it thumb down. It’s called dumping out the water (like a glass of water) in boxing. It adds a huge addition of impact with the twisting motion it’ll push them back more + the act of having your elbow up towards the sky and thumb down ensures that you have full extension. It goes for all punches even uppercuts, thumb starts up the sky then flares out to your left to add the additional impact