AHBA HCT. Leg 1 is purely an instinct test. The dog doesn't need to have been on stock before. It is helpful to have some obedience to stop the dog. The evaluator will tell you anything you need to do. Unless the evaluator tells you to correct your dog DON"T!!!!!! What you think is horrible and terrible and a take down, just may not be. If you correct a soft dog when it is only showing instinct, you can inhibit the dog. You might be in the pen with the sheep, or not. Wear sturdy shoes that you don't mind being ruined. Bring a flat collar in addition to any collar you usually use.
Leg 2, is very similar to the AKC HT. You show controlled movement of stock around some cones, a stop of ANY kind- stand, sit or down and then a recall. Recall is not the formal obedience recall. You can be only a few feet away and no front is required, just the dog come to you and you grab it before he decides the sheep need more movement
In AKC and AHBA, you can start the dog at any level- you can do advanced if you think you can do it. AHBA and AKC have no relation and no agreement on prereqs. A PT is pre-trial test. It is above the HT and below Herding Started (HS). You don't need a PT or HT to do trial level.
I gotta go and practice herding- have all three dogs in an AKC herding trial this weekend.