Drag a cog from the tray until it clicks into the teeth of another. Two meshed cogs turn opposite ways.
Speed
A cog with half the teeth turns twice as fast. Cogs in between do not change the speed — only the two ends matter.
Direction
Every cog in a chain flips the direction. An odd number of cogs between motor and machine keeps the motor's direction; an even number reverses it.
Jams
If a cog is told to turn two different ways, everything locks. Three cogs all touching each other is the classic way to seize a train.
Decks
Some benches have two decks. Cogs only mesh with cogs on their own deck. Drop a cog on top of another to share its axle — they turn together, and that is how you change speed.
Belts
Pulleys don't mesh. Belt two of them together to carry drive across a gap. A belt keeps direction; cross it to reverse.
Ratchets
A ratchet only turns one way. Drive it backwards and the whole bench seizes.