Great thread and some great posts.
I want to piggyback on some of the things that Allison and Kayty said.
It's important to be very precise in your terminology; collection, on the bit, on the aids are used to mean a variety of things to a variety of people and disciplines and it contributes to the misunderstanding and confusion.
I dislike the terms "on the bit" and "in a frame" because it encourages the idea that you acheive it by riding the bit or the horse's head. I much prefer the term "on the aids." The second photo Allison post above shows a horse nicely on the aids - moving from behind, *connected* back to front, round through the back, attentive and obedient. You mostly feel this through the horses back; it rises underneath the saddle and suddenly the horses' rhythym is very clear. Your contact through the reins is more distinct because the horse has "picked up the phone" on his side of the conversation. Where the horse's head is positioned is incidental at this point; if the rest is correct, the horse will likely have flexed at the poll and brought his face closer to vertical, but really, it's the last thing to consider. This is the useful place from which all good quality flat work comes.
"Collection" is further along in the process. "On the aids" is the foundation for developing collection, but there are a lot of other building blocks on the foundation before you can get to collection: fitness and muscle ability, inside leg to outside rein, and the rest of the training scale - rhythm, suppleness, contact, impulsion, straightness. You can consider "on the aids" pre-collection if you wish.
Collection occurs when the horse "sits" behind, increases the angulation in all the joints of his hind limbs, shifts his balance rearward, and lengthens his topline while shortening his bottomline.
That some trainers muddy the waters further by calling on the aids "collected" makes it even harder for a riding student to understand.
Doesn't necessarily mean that they're bad trainers or instructors. However, if anyone attempting to teach these concepts starts out with the position of the head or how to ride the head and neck - RUN AWAY!
ETA: Agree with Spyder ^^ For trail riding and general riding, you want a horse that's connected, or on the aids - he'll move and cover ground more efficiently, and be more of a pleasure to ride.
End rant, sorry.
Last edited by maura; 05-29-2011 at 08:05 AM.