Metal drags are made entirely of metal in the shape of a "C". This type of drag is excellent for smoothing-out "washboarding" of dirt roads (caused by yahoos driving too fast on them). The best type of drag is constructed of used tires bolted together and attached to a metal bar; the more tires the better. A six or an eight tire drag is superb for producing a glass-like appearance on the road. When towed at a speed of about 5 mph, the tire drag produces an appearance on the road that has no equal when it comes to the initial sign cutting.
The Border Patrol still uses drags to perform the necessary function of clearing old and already-chased sign from The Line and from other roads in the interior. The dirt around head gates on canals are simply raked-out with a common metal rake to create a clean slate for new foot traffic.
Once the road has been dragged the sign cutting (looking for sign) actually begins. As much as possible, it is desirable to cut into the sun. This way, the shadows created by the sun on the side of the sign facing the sign cutter make the sign stand out like a billboard.
Some Tonks attempt to obliterate their sign across a drag road by dragging a branch filled with leaves behind them. The brush-out shows up as well as the actual footprints. All the tracker has to do is to follow the brush-out until the Tonk has dropped the branch. Then, his or her sign will be recordable. For example: "I'm following a group of six that brushed-out two miles East of the Port of Entry (POE). They're 3 diamond tennies, one plain flat and a broken-line irrigator boot".
Armed with such information, Agents farther in the interior can begin looking for the same sign upon known avenues of travel.
There is much experience required to be a tracker and no one person is perfect at the endeavor. Some, though, can come fairly close to perfection. The Agent who taught me, Eli, was known for his ability to cut sign across malapai. One of the Old Patrol has written a book on the subject. His name is Joel Hardin and his book, Tracker, can be found on Amazon (the last time I checked).