Its very strenuous. He will have to take as deep of a breath as possible & blow out as fast & hard as possible until there is nothing left (he might feel lightheaded/weak from doing this) & if the person doing the pft's with him is anything like my office they will be yelling/encouraging by saying:
QUICK & HARD, BLOW, BLOW, BLOW KEEP GOING, THATS IT, MORE MORE MORE TO THE LINE (for me its that for a 5 year old it might be a candle picture of something), GO GO, ALMOST THERE GO etc
BTW....for the adults I AM referring to the pfts during all these comments. As I am typing I kept thinking of my husband LOL! OMG!