Warm air is produced when you open your throat and lower jaw. You can feel the difference if you put your hand in front of your mouth and blow with your mouth open wide and throat open like when you say "AAWWW" compared to when you pucker your lips like when you are going to whistle or say "OOO." You can feel the warm air on your hand. When you play with your throat open and you are thinking "AW" you will be blowing warm air. Warm air produces a bigger, fuller tone. That's what I mean.