For me Slipping is where you remain in contact distance but make them miss allow yourself the instant retaliation.Your head moves sure.

Actual head movement as in 'hard to hit' cause its on the move and not easily read, stems from many things cause your head will follow your feet your ankles your knees your hips your neck even your upper torso twisting will get your head out of the way of a straight shot as will just turning both ankles out or in one way or bending a knee.

I think people who have no neck and whos head follows their shoulder movment are easier to read so they have to stretch and lossen all the other joints mentioned and add it into their footwork so as to make up for it.Lacy lacks in that.Tyson utilized it and made it a strength by going low and head hunting from low down by covering up leaning and swaying in low with his knees and feet;(tough if you get timed and caught doing that though).