Its to do with turning the hook over as the hookling slides over the bottom of a carp's mouth when the fish moves away from the lead and the link tightens - so the hook-point is facing downwards rather than whole hook possibly being dragged across on its side over the carp's bottom lip, failing to prick into some flesh.
Try it with the "across the back of the hand" technique with rigs set up with and without the tubing..... - although this isn't an essentially accurate test (not under water for a start!) maybe it can help you see the mechanics of it.....
Imagine the back of your hand is the bottom of a carp's mouth - and that the carp has sucking in your bait and hook and is moving away.... the hooklink slides across the surface and see if the shrink tube turns the hook up
