The diaphragm doesn't help you breathe out, it helps you breathe in. When you're told to use your diaphragm when singing or playing an instrument, what is meant is to put some power into the air; use your lower ribcage to empty your whole lungs, rather than just your upper chest to give feeble puffing.
Imagine that you're squeezing yourself out like a tube.
And when you breathe in, try to push all the air into the bottom of your chest until the top fills up: far too many people snatch breaths and take and use half-lungfuls of air.