A bull put spread involves being short a put option and long another put option with the same expiration but with a lower strike. The short put generates income, whereas the long put’s main purpose is to offset assignment risk and protect the investor in case of a sharp move downward. Because of the relationship between the two strike prices, the investor will always receive a premium (credit) when initiating this position.
This strategy entails precisely limited risk and reward potential. The most this spread can earn is the net premium received at the outset, which is likeliest if the stock price stays steady or rises.
If the forecast is wrong and the stock declines instead, the strategy leaves the investor with either a lower profit or a loss. The maximum loss is capped by the long put. It is interesting to compare this strategy to the bull call spread. The profit/loss payoff profiles are exactly the same, once adjusted for the net cost to carry. The chief difference is the timing of the cash flows and the potential for early assignment.
The bull call spread requires a known initial outlay for an unknown eventual return; the bull put spread produces a known initial cash inflow in exchange for a possible outlay later on.