When you sell a put option there's a margin requirement, regardless of whether it's a Naked Put or Cash Secured Put, or if you're trading in a margin or non-margin account. The only difference is that in a non-margin account, like an IRA, the margin requirement for a Cash Secured Put is 100%, meaning that the full amount of cash required to buy the underlying at the strike price, if assigned, must be held in reserve.
When you trade Cash Secured Puts (CSP's) you have a Cash Balance and Option Buying Power (i.e. cash available to trade). Forgetting about premiums for a moment, when you sell a CSP, the Cash Balance doesn't change, however the Option Buying Power is reduced by the full amount of the strike sold. When you close a CSP (i.e. buy it back), the Cash Balance also doesn't change, however the Option Buying Power is increased by the full amount of the strike previously sold, since it's no longer required to secure the put. The premiums received or paid actually affect your Cash Balance.
In the following example, I used Thinkorswim's PaperMoney account to simulate a CSP trade. The initial Cash Balance and Option Buying power was $100,000.
After selling a CSP on SPY with a strike of 445, the Cash Balance was increased by the amount of premium received ($100,000 + $495.33 = $100,495.33), and the Option Buying Power was reduced by the amount required to secure the put ($100,495.33 - $44,500 = $55,995.33).
The CSP was then rolled out to a further expiration and rolled down to a lower strike of 435 for a net credit. The Cash Balance was increased by the credit received (100,495.33 + 130.67 = $100,626.00), and the Option Buying Power was also increased ($100,626.00 - $43,500.00 = $57,126.00), since the amount required to secure the new strike is lower than the amount required to secure the initial strike.
So, for CSP's, only the premiums received or paid affect your Cash Balance, and the sale or purchase of the CSP affects your margin requirement, which also affects your Option Buying Power. By rolling down you are reducing the amount of margin required, and thereby increasing your Option Buying Power.
