I'm switching from Scottrade to TD Ameritrade next week. With most brokers, there's a 3 day settlement period with the funds you trade with. Using the same funds, in the same day, you can buy, then sell, then buy again, but that's all for the next 3-4 days. So it was always safest to wait 3 days after selling any stock before buying using the funds again. If you did happen to sell the stock within this 3 day period, it's considered a free ride. First time you get a warning call. 2nd time, they place restrictions on your account.
If you are trading in a cash account, and you have margin, this restriction doesn't apply. Also you can leverage up to 100% with margin.
Now comes me. I'm trading out of a traditional IRA account. You can't run margin in an IRA account because the government doesn't allow you to put up the IRA balance as collateral.
Along comes TD Ameritrade. They allow you to ignore the settlement rules in your IRA account. You can buy and sell using the same funds all day long, by using their money, as long as the value of the securities you purchase is never more than the funds in your account, and you have a minimum of 25k in the account at the beginning of the day. The only fee differences is you pay 9.99 per trade instead of 7.00. And the first 60 days comes with no trading fees.