I think learning how to do an auto release is really less on where to keep your hands and more learning how to maintain a constant contact with your horse's mouth over the fence. Assuming your position is strong and balanced enough to support our body. FYI- as much as George Morris harps on the importance of learning an auto release (and tells all but the beginner jumpers in his articles in Practical Horseman), I've seen a lot of his clinics and the classes I've seen where he's taught it have usually been his 4 ft class (I think he started on it in one of his adv 3"6 classes). I have no idea where your riding skills are or how secure your position is, but I'm just mentioning it because if you aren't 100% ready, your horse's mouth will be what suffers. Also, before you start thinking about contact over the fence, make sure your contact on the flat is solid first. You might be an absolute pro at that, but I just thought I should throw that out there!
That being said... a short crest release is when you have contact with your horse's mouth over the fence, but are still resting your hands on top of your horse's crest. GM teaches the auto by telling his students to maintain the contact, but drop your hands a few inches down still pressing on the sides of your horse's neck. until they are more stable. Then eventually you won't need your horse's neck at all.