To power your voice requires effort. It is not a passive thing. Air must move and air must be held back (breath support/breath control must be applied and balanced). This compression (squeeze) effort can be subtle or quite strong. And there is a catch-22: Squeeze at the wrong place, at the wrong time or for too long and you'll end up in vocal trouble.
Where To Squeeze:
You should sense your power coming from below the belt...your pelvic floor, or saddle area. You can even sense it coming your heels or thighs. You can also balance yourself there by what you do with your hand on the mic, instrument or just with certain hand gestures. CAUTION-- that hand action must not cause tension in the arm higher than the elbow.
This squeeze can be quite subtle for easier vocal passages... really just staying flexibly tall with your head balanced back over your tailbone can automatically provide all the balanced power you need until you come to a harder place. Then you should rev up the power ... but only from the right places (from below the belt, and from using your hands).
DO NOT squeeze from your ribs, neck, jaw or shoulders.
When To Squeeze:
When you need to apply a lot of power, apply greater tension from the appropriate places. For instance, you might press against a piano stool and press your feet down to apply a power surge.
Do NOT keep tension in the butt, pelvic area or thighs... this will cause tension to move up where you don't want it and will tire you out so there's nothing left when you need more. A great trick to learn is to quickly DE-TENSE at every opportunity. You should look both relaxed and full of energy if you're doing this properly.
Make sense to you?