They do it thru trigger trades.  Or "if this, then that".  In other words, you put up and order with another order behind it, so it is constantly filled.

You won't find them much except on special brokers.  you TDA's won't have but one, but there are other brokers where you can pay a monthly fee, usually around 100 dollars, and have unlimited trading.  But if you meet mi imum number of trades per month, around 25-30 usually, they wave the fee.

They wave the fee because on every trade is a built in commission that is invisible to you, that's on top of the commission you see from your TDA's.  That commission is shared with the governing bodies and the broker.