Belt slipping
The motor is on a carriage that moves back and forth with springs keeping tension on the belt. This isn't just to keep the belt properly tensioned, it also acts as a clutch to protect the motor from being overloaded. When the washer goes into spin, the motor needs to get up to full speed as quickly as possible to avoid burning out the starter coil, but a big tub full of water and clothes cannot accelerate to 618 rpm that quickly, therefor there needs to be something to serve as a clutch between the motor and tub to allow the tub to slowly ramp up speed with the motor already at full rpm.
What happens is the motor will start and immediately go to full speed, but the belt will be unable to do so. This will pull the motor on its carriage closer to the center, loosening tension on the belt which allows the motor pulley to "slip" or spin freely while still applying a force on the belt. As the tub picks up speed, the force pulling the motor towards the center will weaken, allowing the springs on the carriage to pull the motor and tighten the tension on the belt, increasing the force applied to the belt and accelerating the tub even more. Eventually, the tub will reach full speed and the belt will stop slipping on the motor pulley.
This is how it's supposed to work, and it's quite ingenious actually because it gives the drive assembly the functionality of a clutch without using an individual clutch part. If you use cheap belts that aren't designed to slip like this, or the pump and drive belts aren't tensioned together properly, the drive belt will be unable to slip on the motor pulley like it is supposed to, and the motor will overload and smoke.
Hope this explanation helps.