Pilates principles of control, alignment, breathing and body awareness can cross over into performing gym exercises too. However, instead of pumping out reps and sets of plates/ dumbbell/ body weight exercises, Pilates exercises are performed at a slower and more constant pace for longer endurance sets (eg 20-30 repetitions). This is to encourage control and endurance within all muscle groups.
From a physics perspective, using springs varies from using “weights” because a spring will change its kinetic energy dependent on how far it is pulled out and therefore, is not under a constant and unchanging amount of load or force. This causes the body to have to constantly adjust to the imposed demands of this changing load. Also, working out on moveable equipment adds an extra stability challenge, forcing the body to have to recruit deeper stabilizing muscles (core) to keep the body in the correct position.