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. This is the case for most maps of the DDmaX category.
As the hook lasts only 1.25 seconds, a rehook is needed to keep the tee hooked.
Edge hook with speed (Speededge)
This is a technique where you take speed off your partner (or dummy) in order to travel enough distance.
The edge-rehook is needed to travel that distance.
Edge hook with grenade (Rocket-Edge)
A Rocket-Edge is essentially using the grenade launcher to get to a surtain position in order to be able to edge-rehook and travel the distance that is needed.
You can find these kind of parts on a higher difficulty map.
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.