Three versions of the Index of Coordination are shown in the following image.
In the middle image the entry arm begins to generate force at the same time that the opposite arm stops generating force. The Index of Coordination (IdC) is zero and the arms are in opposition.
In the top image the entry arm remains motionless in front of the body while the opposite arm begins the recovery. The IdC is negative and is commonly referred to as “catch-up stroke”
In the bottom image the entry arm begins to generate force before the push is complete. The IdC is positive and also called superposition .
Using the definitions of the different types of IdC, the effectiveness of each of the three coordinations becomes apparent. With a negative IdC, no arm is generating propulsion for a fraction of the stroke cycle. With a zero IdC, one arm is always generating propulsion. With a positive IdC, both arms are generating propulsion for a fraction of the stroke cycle. It’s clear from the definitions that a positive IdC will produce more constant propulsion, and therefore, the fastest swimming.