If you get off the treadmill and lift the belt off the deck with your hands there should be about 3 to 4 inches of give. Jerking means there could be too much and the belt is slipping. Tighten the belt using the allen wrench that came with the treadmill when you bought it. By the same token if it's too tight then loosen it.
Your treadmill belt might just need to be centered. With this you use the same allen wrench and turn clockwise the roller bolt. You do this until the belt is centered and then allow the belt to run by itself a few times and you'll be able to see if it's turning without moving width ways. If you can't locate the allen wrench then a hardware store will have one you can buy.
If the machine moves even just slightly it maybe that you have slippery floors. Use a treadmill mat to put the machine on and it should stay stable in one place. If all of the above hasn't solved the jerking problem or the treadmill is still moving violently then we would recommend you stop using it and unplug it from the outlet.
Call for a professional technician to come and have a look to see what's going on if you've tried everything. Their number can be found online or in your manual, if you explain the steps you've taken to solve the issue then the technician can perform some of his own diagnostics to see if he can find the problem.