This represents (among other things) the relationship between pressure and temperature. If you assume volume is a constant (the sealed canner) and just ignore the 'nR' bit, it's obvious that if you raise 'P' that 'T' will also go up. It will go up just as high as 'P' goes, unless some other part of the equation changes. The point where 'V' changes is determined by how heavy the vent weight is. When the pressure inside the canner becomes great enough to raise the weight on the vent, that will directly determine how high the temperature can go, i.e., all home canners I've seen regulate the temperature by regulating the pressure. Simple, and provides an automatic safety backup to the design, even though pressure cookers & canners are also provided with safety blowout plugs, too.