Analogue testing was developed by Alan Sales. He noticed that when he tested a muscle through the range of its motion (from contraction to extension), there would sometimes be a ‘blip’ in the smooth change of position of the arm or leg being tested. Alan saw this as representing how locked or unlocked a muscle was. So instead of having 0 (unlocked) and 100 (locked) for a muscle test, he could now get a percentage. 
This allowed for a more nuanced approach and was a way of finding imbalances that the binary system might not reveal. It would be unwieldy to have to test every muscle in this way, so Alan identified a finger mode that could be used instead. When this mode is in circuit, the muscle will unlock if the circuit is anything from 1% to 100% out of balance. 
Alan used the analogy of digital and analogue format in audio. When you listen to a piece of music in digital, all the sounds outside the set bandwidth are deleted. It is either there or not there. In analogue, you hear the sound at the bandwidth and also the sounds close by.