The "trading" is all out of band, the system only sees one person drop their registration and another person fill the slot. When someone drops a class, the ope

The "trading" is all out of band, the system only sees one person drop their registration and another person fill the slot. When someone drops a class, the opening becomes available immediately, so you coordinate a time well after the registration rush has died down for one person to click "drop" and the other to refresh the page and click "register". At least that's how it worked when I did it 20 years ago. It was relatively common in the Greek system not to "trade" a class for a class but rather a class for a few beers or the like: prior to registration, if you were an underclassman who really needed to get into a class next quarter that you knew filled up quickly, you'd find an upperclassman (who get access to the system earlier) who was eligible for the class you needed and wasn't full on credits to grab a slot, then a couple days before classes started, you'd have them drop and then grab it for yourself. At that time (early 2000s), polling bots weren't common, but there were rumors, so people doing this got more careful and actually sat next to each other with their laptops to coordinate the drop and add instead of just picking a time or date.