When the drum inside a top-loader spins to drain the water, it uses centrifugal force to squeeze the water out. This means it spins extremely fast.

For example, this model spins at 710 RPM

http://www.aaxon.com/blog/194-speed-queen-top-load-washer-vs...

However someone else pointed out that if they don't lock the lid, they might engage a brake and stop the drum's spinning if the lid is opened. This achieves a similar goal.