To simulate a freeflow, you or your instructor will depress the purge button.
Hold the mouthpiece against your lips – don’t seal your mouth on it.
Allow excess air to escape; “sip” the air you need from the flow.
It may help to insert just one side of the mouthpiece into your mouth, with the other side out.
Practice for at least 30 seconds.
Check your SPG after practicing. You may be surprised how much air you used. This emphasizes the need to start your ascent immediately when breathing from a freeflowing regulator.