Basically just look at the math. Horsepower is the product of torque and rpm.

To get more power out of a motor without increasing its torque, you have to make it rev higher. The way you do this is make the motor withstand the stress associated with spinning faster.

To get more torque out of a motor without revving it higher, you have to increase its torque. The way you increase torque is to increase the breathing capacity of the motor through any number of ways including bolt-on mods, cams, more displacement, etc.