Having had my rant about what is not correct, I have had people writing in about whether it’s the cannon bones lining up or the cannon bone of the back with the fore limb on the front, and to clear things up a bit I decided to go to the FEI rules on the trot….
To elaborate on my discussion I will quote the FEI rules on the trot and extended trot, with four very nice examples of what is according to these rules correct…
“The expression “good hock action” underlines the importance of an impulsion originating from the activity of the hindquarters.” My interpretation of this is that “good hock action” is not simply up and down hocks, it is a term that stresses the importance of hocks that propel the horse forward, and step under in order to cover more ground.
“The fore feet should touch the ground on the spot towards which they are pointing.” This point is very interesting, this means that as the front foot is reaching forward it should not drop back to touch the ground. The front foot can only touch the spot it is pointing too if the hind leg is reaching forward and pushing the horse through and over the track of the front. A horse with hocks that go up but not forward will force the front leg to drop back before it hits the ground.
“The movement of the fore and hind legs should reach equally forward in the moment of extension.” For me this means that even if the cannon bones are parallel, the horse must be reaching forward by the same measure with the hind leg and the front leg, i.e. there can’t be a massive gap between where the back leg will land and the diagonal front leg with lift off.
So parallel lines aside, activity without engagement, is just going up and down on the spot. Engagement without activity is far more correct, and engagement with activity is the icing on the cake, which of course takes patience, correct training, and a rider’s ability to be humble, listen, and wait.