Performing the Glitch
The way this glitch works is by tricking the starting timer off of killing the dummy.
Originally found last year on reddit by /u/ficoplati. So thanks to him for that.
Throw an airstrike marker at your feet.
Shoot the dummy three times.
Get the airstrike marker to spawn again, come close to the dummy, and throw it at your feet. The goal is for you kill yourself and the dummy at the same time.
Respawn around the first tick happens, and then just go through the course like normal, but don't cross the finish line (either of them).
You'll know it worked if each dummy on the actual course is down.
Without crossing the finish line(s) stand kind of close to the gate, and start throwing airstrike markers at the general location of the starting dummy.
General location of where to throw.
After killing the dummy, wait for the countdown to end and cross the gate.
If you are interested in doing this glitch, but don't want to just follow a text guide, you can check out a video!