They do it thru trigger trades. Or "if this, the
Post# of 96878
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.