Quote from cstfx:
Say you have 20 traders each put up 50k deposits (for simplicity sake). That gives you 1MM on deposit, 4MM buying power using intraday margin rules.
Now, you give each trader 20:1 leverage on their 50k, that"s 1MM buying power (BP) each. Times 20 traders, isn't that 20MM BP?
Well, yes and no. Each trader has their own limits but the setup is that it would be extremely rare that all traders would use all of their equity at the same time, thus overeaching the main accounts trading limits. Plus, you have trades that would offset each other having no effect on the main account margins. You may be long 1000 shares of Citi, but another guy is short the same amt, thus crossiing out the trades (for margin purposes on the main account)
Generally a prop owner will have to put up some of his own money to protect his group in the event that this marging of the main account is breached. There are groups out there that don't back their group's trading with any of their own funds, and these are usually the horror stories you keep hearing about.
* this is a generalization about non-licensed prop firms since the main account that a prop uses is nothing more than a retail trading account which uses money management features to control each individual trader's accounts.