The most basic power-transmission device is the shaft. Nothing fancy, just carries the power from one place to another, with no changes to either the torque or the speed. Pay attention, though, to the direction to avoid headaches.

Gearboxes are used to change the ratio of torque to speed, while keeping the power constant. They come in 4 ratios - 2:1, 4:1, 8:1, and 16:1, and will affect the torque and speed accordingly. Reduction gears increase the torque and decrease the speed; acceleration gears do the opposite. Be warned: these need to be lubricated to work at their full potential; unlubricated gearboxes will gradually wear down, and reduce the transmitted torque accordingly.

Bevel gears are used to change the direction of power transmission without changing the torque or speed. Unlike their inline cousins, they do not need lubricant, and will operate continuously and losslessly.

The shaft junction does exactly what it sounds like. In 'merge' mode, it will take two engines' outputs and combine them, outputting the sum of the torques at the input speed. Note that trying to combine inputs of different speeds will merely result in a shower of sparks. In 'split' mode, they will take an input and send some torque each way, at the input speed.

The clutch is basically a gated shaft - when powered by redstone, it transmits its input torque and speed unchanged. Unpower it, and it outputs nothing.

The dynamometer will display its input torque, speed, and power while outputting them to the other side.

Flywheels store rotational kinetic energy. They take some time to spin up to the input speed, but when they do, they keep spinning for some time, providing a more steady power output to a machine than a shaft hooked up to a varying input. Heavier materials make better, more efficient - and more powerful - flywheels. Spinning one of these too rapidly will result in a violent and destructive failure.

The worm gear is a fast way to increase shaft torque without using a large number of intermediate gears, creating an equivalent 16:1 ratio. However, worm gears are inefficient and will lose some of their power, leading to a much slower output speed. This loss rate goes up as the input speed does.

A CVT (continuously variable transmission) is essentially a dynamic gearbox. Rather than using toothed wheels, it uses a series of belts to change the gear ratio. This gives them the advantage of being efficient and highly flexible, allowing you to select the desired gear ratio on demand. Note that higher ratios require more belts, with each power of 2 doubling the maximum ratio (capped at 32:1).

The industrial coil stores energy in a large spring that can be unwound on command. Due to the fact that its output torque and speed can be chosen at will, it can be used to store energy for later or as a sort of capacitor, to store low-power energy accumulated over a long time and then release it in one powerful burst.