And regulated - you have to be approved for margin trading by the brokerage.
So for example, if I was using an 100k account to trade, I had another 100k of leverage no?
I note that Etrade at least implements it less as "margin approval" and more as "advanced mode"; to do any sort of options trades, even the ones that don't require margin themselves, requires you to activate the "margin feature" on your account.
It also is not a straight 1:1 ratio. You generally can't go absolutely nuclear with leverage unless you have portfolio margin. Typically, brokerages will require collateral in the account equal to the risk (for defined risk strategies) and you will need a higher trading clearance on your account for unlimited risk.
Your buying power is a function of how long the trade will be held as well as the risk profile.