Complex sentence: The engine speed is sensed by a pair of weights that rotate with the crankshaft, as the speed increases then centrifugal force causes the weights to move outward against the pressure of a retaining spring.
Split into simpler sentences: The engine speed is sensed by a pair of weights that rotate with the crankshaft. As the speed increases, centrifugal force causes the weights to move outward against the pressure of a retaining spring.