Wiping away the cobwebs, having been a pupil and an instructor on Harvards (AT 6 Texan if you must) I recall the following. Bear in mind this is a tailwheel aircraft. There were four reasons why the aircraf yawed to the left when power was applied for take-off.
1. Torque effect ie the engine wants to turn in the opposite diection to the propeller.
2. Asymmetric thrust caused by the lift produced by the down-going blade.
3. Precession caused by the tail coming up, the force being applied at the top of the propeller and taking effect 90 degrees later (clocwise rotating propeller when viewed from the cockpit).
4. Slipstream effect caused by the slipstream of the propeller flowing around the fuselage and striking the vertical fin on the left (similar to a corkscrew). It is for this reason that the fin is offset by two degrees when viewed from the rear.
In S&L flight the nose also pitched up when power was applied.
The converse applied when the power was reduced ie nose pitch down and yaw to the right.