If your treadmill is plagued by a jerking or slipping motion, you likely have either too much loose tread or your running belt is not properly centered. When either of these things occur, the belt will feel choppy and unbalanced. Both issues can be fixed with a quick and easy process.
Find your treadmill's belt adjustment bolts. Generally these are toward the back of the treadmill, on both sides of the end of the belt.
Set your treadmill at a slow walking pace of 2 or 3 mph. You will need to have the tread moving as you adjust it to tighten or center the belt.
Find the correct size Allen wrench for your treadmill. These hexagonal wrenches normally come packaged with the treadmill, but if you have lost it or you bought the treadmill second-hand, buy a ring of different sizes of Allen wrenches at any hardware store.
Insert the Allen wrench into the bolt on the right side of the belt and turn it clockwise for one-quarter turn. You should see the right side of the belt pull slightly tighter as you turn the wrench.
Insert the Allen wrench in the left bolt and turn it clockwise for one-quarter turn.
Walk on the treadmill for a few minutes to see if it still jerks. If so, you may need to tighten up both adjustment bolts again to remove any excess loose tread. If the belt seems to be favoring one side, tighten only that side to see if you can re-center the belt.