I think that many people have different definitions of on the bit, and I also know that your understanding of on the bit can change pretty dramatically as your feel and education increase. I'd like to share some of my granulations of the concept to share what I've learned in the last year.
Your mileage may vary, of course.
In a frame is when the horse's nose starts to drop, closing the angle behind the poll. Sometimes this is a 'steady frame' (ie the height of the poll isn't changing, nor is the angle behind the poll) and sometimes it isn't.
Round is when a horse is taking steps under their body with their hind legs, and lifting their back.
On the bit is when you close your leg and the horse goes to the bridle, or when you reach forward with the bit and they follow it where you put it. In my mind, on the bit is both in a frame, and round, and a little bit more.
I see a lot of management of 'frames' in horses, especially in green horses who haven't quite figured out how to balance their steps without flinging their head about, but sometimes even in school horses with riders who sort of forget to keep a lid on the connection.
I think that frame-management starts to disappear as the horse becomes rounder, using their body in a healthier manner and controlling their balance, but you can still sometimes see riders with quite round horses go to push their hands toward the bit and ask for a stretch, and it all goes out the window with the horse shoving their nose up and against the hand.
A horse that is truly on the bit has activated all of the seeking reflexes and is bringing the back up into the rider's seat, is reaching from the neck into the bridle, even within collection.
Cool, right? Sounds awesome. I want my horse seeking the bit, softly swinging over his back, eager to reach downwards and forwards when I close my leg and gently push my knuckles forward. I'll discuss what helped me and Moxie with this concept the most in the next post, but I am curious - what are your thoughts on the distinctions I've laid out above? Do you use the terms differently, and if so, how do you define them?