How about this...

They make their commission on the spread.

Lets say a seller put's in an order to sell at .10, they scoop it up, then create an ask of .105 and someone grabs it.

They also try and drop the price trading back and forth amongst themselves, such as when you see 100 going for .095 like happened I think yesterday, in an attempt to shake more loose from a "nervous nelly" then bump up the ask.

Far more complicated than this, not 100% accurate, but plain English.

Does that help?