Fleming's Left-hand Rule is visual mnemonic created by John Ambrose Fleming, in the late 19th century, as a way of working out the direction of physical force in an electric motor.
When an electrical current flows through a copper wire, it generates a small magnetic field that rotates counter-clockwise around the wire. When the wire is place within a larger magnetic field, the wire will be physically pushed outside of the field due to wire's magnetic field running in opposition to the larger magnetic field on one side (due to the wire's magnetic field moving around the wire, and thusly changes its directional pull, while the larger magnetic field simply runs "north" to "south"). The Left-hand Rule can be used to predict which way the wire will be pushed, with the forefinger positioned to mimic the larger magnet field and the secondary finger positioned to mimic the position of the copper wire. With all three digits positioned perpendicular to each other, the rule states that the wire will be forced in the direction of where-ever the thumb is pointing.