The clutch bellcrank transfers the pressure from the foot petal to the Chevrolet V-8 clutch throw out bearing. This forces the motor driven disk rotor to press against the friction material between the motor rotor and the front rotor fastened to the driveshaft. It requires 120 pounds of pressure on the throw out bearing to fully engage the clutch. 80 pounds will operate the hammer at the slowest speed.
The foot petal gives a 5 times leverage increase. So, 20 pounds foot pressure is required for full clutch engagement.