It is derived by knowing that 1 horsepower is 550 ft/lbs per second.
RPM's are broken down into radians per second (2 x 3.1416/60) = 0.10472
Foot pounds are then divided by the radians per second. 550/0.10472=5252
All engine dyno charts should show horsepower and torque curves crossing at
5252 rpm or they are wrong.
SAE HP at the crank is crap..it's just a way for Car mfgs to exagerate the Hp of the car.. who buys a motor just to run it while it is not connected to anything? a motor does work. They all should be rated at the place where the "Rubber meats the road"