It is an interesting comment on how a person reacts to, let's say a loud sound, at a moment when he or she is focusing on something in particular. In other words, being interrupted.
Milt said he would hate to see the animated character turn right away toward the direction of the disruption. That way the audience is missing a "thinking beat".
His way of dealing with a moment like this one is having the character straighten up (after a squash), change expression according to how the character feels, THEN animate the head turn in the direction where the disturbance came from.
And that's exactly what Medusa does when she hears a loud noise coming from upstairs, where her alligators are chasing Bernard and Bianca.
Medusa is studying a map before reacting to the commotion. Notice how Milt always keeps her eye in the clear among the messy hair shapes. The expression change happens at the beginning of her head turn, where the audience can see it clearly.
Of course there are exceptions to this "rule", but before breaking it, it's useful to know the rule.
Looking at these drawings, it is obvious that Milt's Medusa is a revolutionary invention.
