SHP is worked out by Tq X RPM divided by 5252. So in reducing the RPM, you can increase the torque and produce the same SHP.
This always made me wonder. Say you left the RPM at the max setting and then increased the throttle to the maximum torque figure for the lowest RPM setting, other than having the engine output more than its book rated SHP what is the harm being done? No particular component is working harder* than it otherwise might have?
*or should I say beyond its limits.