I throw off the hump most of the time and have found that if you want an commercial clay tool, the 'nylon clay cutter' from Kemper Tools is what I usually use. It is like their wire cutting tool which comes with two wooden toggles on the ends, but has a soft braided nylon string instead of wire. When I buy one, I cut it in half in the middle of the string so that I have two matching string tools. I hold onto the wooden part and the loose end of the string wraps around the clay on the wheel (in the cut line that I have marked under the pot with a wooden tool or rib - like Pres said) to cleanly slice off the pot. I rarely have to tug the string, just let it wrap around and pull gently to slice straight through the bottom.
Now that I think of it, I believe that there is a Van Gilder string tool that is specifically designed for this as well.