You should hold the bucking bar lightly but firmly against the end of the rivet shank so as not to unseat the rivet head. The inertia of this tool provides the force that bucks (upsets) the rivet and forms a flat, headlike bucktail. A satisfactory rivet installation depends largely on the condition of the bucking bar and your ability to use it. If possible, hold the bucking bar which will allow the longest portion of the bar to be in line with the rivet.