The tight end's "drag" route is defined as "drag 3-5 yards, if no ball sit 3 yards from sideline" (via the Shanahan 2004 playbook). The tight end "drag" route should not be confused with a "drive" or "cross" that has also been referred to as "drag."
The outsider receiver runs a quick three yard vertical stem and breaks at a 45 degree angle inside. He can convert this slant to an "in" if a zone dropper occupies his original destination:
The primary read is the drag. The secondary read goes to the slant/in and the half-back (H; above) acts as the third/checkdown read.
A slight variant of "dragon" asks the outsider receiver to make his initial break at 5-8 yards (called "Drag Slant").
Outside References
2) Pass Concepts
3) Josh McDaniels and the Slant-Flat Concept in Superbowl XLIX