Whenever a current carrying conductor is placed in a magnetic field, it experiences a force. The relation between these three is stated by Fleming’s left-hand rule. To understand this rule, the thumb, middle and index fingers are placed in perpendicular to each other.
Imagine a current carrying conductor placed in-between a horseshoe magnet. This magnet produces some magnetic field which will be in the direction indicated by the forefinger or Index finger. The current passing through the conductor will be in the direction of the middle finger. This conductor experiences a force which is indicated in the direction of thumb. In case, the poles of this magnet were reversed, the applied force will also be in the reverse direction.
The same rule is used for motors. Under a magnetic field, if a current carrying conductor is placed and moved with certain speed, it produces some magnetic field which will be against the magnetic field that is already being applied there. Now, the newly produced magnetic field pushes the conductor. This principle of Fleming’s left-hand rule is applied in electric motors.