And if you don't you've committed a good faith violation and if you do it more than once they will put you in time out and you can't touch your $$ for awhile.
You should look up pattern day trader rules and pattern day trader strategies. Also look up the good faith violation I mentioned above. Both will help explain the whys and hows and keep you out of trouble. Large brokers like Fidelity, TDA and Schwab won't let you get away with rule violations. They'll warn you once but they will lock you out if you don't follow the rules.