Over/under cable coiling refers to a method of storing cables that preserves the capacitance and common-mode rejection ratio built in by the manufacturer with a twist in the cable, and the shielding that encases the twisted pairs within. It also keeps knots to a minimum, allows the cable to lie flat when uncoiled, and makes running the cables easier and faster.
The "over/under" name refers to the practice of twisting the cable in one direction to make the first coil, and un-twisting it to make the next, and repeating this until all the cable is neatly coiled. Connecting the ends on the outside of the loops, or tying them in that position, ensures that the ends don't pass through the loops in storage so there are no knots when the cable is laid out.