 easiest method is to push half the breath out suddenly through pursed lips, a technique taught to asthma patients to increase oxygen absorption. The sudden push momentarily increases intra-lung pressure, which also pushes down the diaphragm and further strengthens the core. There is also a genetically programmed reflex retraction of the upper abdominal wall. More difficult, but far superior, is to learn to narrow the throat, for small-hole exhalation, the way of controlling the breath taught in advanced martial arts.
To benefit most from small-hole exhalation, you have to coordinate the sudden push of breath exactly with the instant of greatest effort in a movement, or the point of impact in a kick or punch. Good examples are the “Ki-eee” shout in martial arts, and the closed-mouth grunt of boxers at the moment they strike. Try the grunt yourself now, with your core tight, and feel your abdomen retract further to increase stabilization. Timing is critical, however, and we see many poorly trained athletes who make the forced exhalation before the point of impact. They immediately lose 10-20% of their power.