Complex sentence: Protein strings tend to remain motionless if the eyes are still, rotating vertically if one looks up and down rapidly, and horizontally if one looks right to left rapidly; in either case slowing down and stopping after a few seconds.
Split into simpler sentences: Protein strings tend to remain motionless if the eyes are still. The strings rotate vertically if one looks up/down rapidly, and the strings also rotate horizontally if one looks right/left rapidly, but in either case they slow down and stop moving after a few seconds.