In the case of the zymogen activation coupled to its enzyme catalyzed reaction, the time-dependent slow manifold propagates (swings) through the phase-plane as long as the non-observable reaction is producing E 2 . In this scenario, the c 2 -nullcline swings through the phase-plane almost like a (curved) windshield wiper rotating counterclockwise. The phase-plane solution to the indicator reaction initially follows behind the swinging c 2 -nullcline until it eventually catches it, at which time c 2 reaches its maximum value.