It's simple, people are making more complex then it needs to be, power (hp) is what moves and accelerates your car from idle/stall until you run out of power, power is equal to torque x rpm. The only way to increase power is to increase torque and or rpm. In a street engine acceptable rpm range is somewhat limited eg. Idle to 5000+rpm, leaving a little room to increase power by moving torque up the rpm range eg.. 4500 vs 5000 peak hp rpm but generally leaving torque the main way to increase power through street rpm range.