Drag marks are where your teeth scrape the putty when you’re making your impression. This spoils the impression as it leaves marks which don’t give SnapCorrect an accurate reading of your mouth.
The best way to avoid them is to make sure you insert your teeth into the putty completely vertically, don’t slide them in sideways at all. And then, vitally, you need to leave the trays in your mouth for the full 3 and a half minutes. If the putty hasn’t hardened properly, then your teeth could scrape and cause marks. If you forget to time yourself, just start counting 3 and a half minutes from when you realize. There is no harm in leaving them in too long.