How spironolactone decreases testosterone production in a pcos woman?

It doesn't. Spironolactone is a diuretic that also happens to block testosterone receptors in hair follicles. Since many women with P COS experience some degree of hirsutism as a result of the elevated testosterone common with PCOS the Spironolactone is prescribed to block the receptors to prevent those hair changes. The testosterone production can be decreased by suppressing the ovaries with OCP's.