The essential element that I don't think has been mentioned here about "bucking back" is that you are using an evener!! It will not work as intended with out it. An Evener is pivoted in the center so as one horse moves forward the single tree of the slower horse moves back, hopefully putting him in draft also. Now with a couple of pulleys and a little ingenuity you can rig the reverse effect on the slow horse as well to encourage him to move up, although I don't recommend using them both at the same time! Good Luck and Take Care!