A drag part is a part in a map where you must hook a tee through a tunnel of freeze, typically through corners and hookthrough. Drag parts are very common in maps. Some maps are entirely made up of drag parts, with only a few non-dragging sections.
As the hook lasts only 3 seconds, a rehook is needed to keep the tee hooked.
Edge hook with speed
Edge hook with grenade
If you wait a bit more when rehooking, you can make the tee you are dragging go under 2 or 3 tiles instead of 1, further tiles require a hammer.