The EHA is sort of like how you play tennis with the racket out in front of you in the middle - should the ball come to the left or right, your racket is in the "center" and can then swing either way.

Same with the EHA - it needs to be adjusted in the "center" so that it can in-turn adjust more rich or more lean as it receives a positive or negative electrical current from the analog computer.

All of this is a "balancing act" depending on fuel pressure and the pressures in the upper/lower chambers of the fuel distributor.