ORIGINAL: low@slow
I'm going to put my two cents in by saying that a prop creates three things known generically as p factor. They are differential blade thrust, slip stream over the tail and gyroscopic precession. These three forces act on the plane depending on your flight configuration Without going into a long winded discussion, the right thrust helps compensate for these factors and generally you will have an easier time flying your plane. Its like putting washout in your wings; Don't have to have it, but it makes the plane easier to fly. If you want to know how a plane fly's, read "Stick and Rudder", an old book but probably one of the best on the subject.
P Factor is another name for Asymmetric Propeller Loading.
There are
four turning forces: Torque, P Factor, Spiral Slipstream and Gyroscopic Precession, all of which have different causes, and can act in different directions under different conditions.
In some texts, they are all referred to generally as
torque, which is technically incorrect, but it is sometimes done in the interest of simplicity. I have never seen all four forces referred to as P Factor.