Imagine that you are watching a taildragger from the side. With the airplane in its tail-down attitude, the bottom of the propeller disk is farther forward than the top of the disk.
Now visualize the path of the propeller blades. During the blade's swing around the right half of its arc, it's also moving forward because the prop disk is tilted back. Similarly, that propeller blade moves back as it makes its way around the left half of the prop disk.
Now imagine the airplane rolling down the runway with its tail still down, so that the prop disk is still tilted back. The entire disk is moving forward, but the blades are moving farther forward as they rotate around the right half of the disk than they are when they rotate around the left half. Because the blades move more forward from 12 o'clock to six o'clock, they take a bigger bite of air. That bigger bite means more thrust on the right half of the prop disk than on the left half. That offset thrust tries to yaw the airplane nose-left. That's one reason you need right rudder during the takeoff roll. During climbs and slow flight, the prop disk of any airplane is tilted back slightly from the relative wind, so right rudder is needed to counter the tendency to yaw left.
Now let's see how this applies to the original question. When you step on the right rudder pedal, the airplane yaws nose-right and slips to the left. The relative wind is now approaching the propeller disk from slightly left of the nose. The blades passing clockwise from three o'clock to nine o'clock take a bigger bite of air than on the other side. This is the same idea as the tilted-back propeller, except that now the prop disk is tilted to the right of the relative wind. The bottom half of the prop disk creates more thrust than the top half, so the airplane pitches nose-up. If you step on the left rudder pedal, the airplane pitches nose-down.
These P-factor effects are more evident at slower airspeeds, so be prepared to react to them when you attempt a forward slip on final approach.
