IMHO,
is a matter of breathing. One have to let ki travel within one's body, and exit when the technique is done.
So, if you want your "students" to exhale properly when practicing, make them shout (I don't know why, but it seems one exhales when shouting
) !
I think Senseï only wanted to make us practice that way, to let us find why a bit later, ourselves...
Thanks for paying attention
...