Plenty of Karate instruction resources will tell you to exhale on impact, suggesting that forcing the air out of your lungs at the moment of contact with your opponent (or the Makiwara) increases force.
True, but if not done properly it will simply leave you soft and out of breath.
Enter Ibuki Breathing.
Using your abdominal muscles, you force air out – not all of it – while tightening your core region on impact. This doesn’t come easily or perhaps even naturally, but it gives you two things: power from the core and a sense of connection on impact, as well as protection from a counter from your opponent. After all, getting caught with a soft core after delivering a blow won’t work in your favor.
According to Nishiyama Sensei, breathing in karate is everything : the beginning, the middle and the end.
As Kanazawa Hirokazu (Kancho) shows in the picture, a solid body core after delivering a technique is paramount, and learning to relax partially after Kime (technique fixation) is just as important.
Good breathing, good karate.