Dwell = How long the main valve stays open.
------>= How long air will hit the ball.
------> Can affect the velocity, shot-to-shot consistency, and air use/efficiency.
If your dwell is set too high, there will be alot of push on the ball, but you'll waste alot of air so you'll run your tank empty, quicker, which means you might run out of air when you're playing.
If your dwell is set too low, you might not always get enough of a push on the ball, so the velocity might be low, and you won't be able to raise the velocity by increasing the pressure. Your velocity might also jump around alot, depending on how much your paintballs drag on the inside of your barrel.
You can also think of dwell as how much you pull back the plunger when you start playing an old-style pinball game...if you increase the dwell, it's like pulling back the plunger farther so the ball in both cases flys faster, or travels harder and has more energy. You use a very low dwell setting and it's like you didn't pull the plunger back very far, and sometimes the pinball doesn't have enough energy to start the game and it falls back to where it started without racking up any points for you.
I hope that helps.
Ray