Whirlpool AWE 6519 / P - what's the deal with the ball in the drain?

My washing machine has smelled lately. I looked here and there and I do not understand a little how the ball in the drain works, which in my opinion should close it to cut off the sewage system, I do not see any other purpose of its presence there.
However, after the wash is finished, this ball falls freely and does not close anything. There is no spring under the ball, so in order for the drain to be closed, water would have to remain in the drain for the ball to float in, but the water does not remain as the washing machine usually pumps it to the last drop.
Or maybe there should be a spring under the ball that something has eaten me during these 8 years of using the device?

Can someone please explain to me exactly how this should work? Should there be a spring there or not? Tomorrow I will definitely install a check valve, such as for washing machines, at the entrance to the sewage system, but I would prefer this ball to work, because even the residual water that remains in the pump and the rest of the drainage system can give off unpleasant odors.

best regards
MS

Hello
The ball is used to keep the powder in the tank, not in the tube going to the pump. Each washing machine has a siphon (pump), so installing any check valves does not make sense ...

I understand you mean a ball in the siphon where the drain hose is connected? ...
Just in case, arranging the hose in such a way that there is water in it, solves the problem of odors from the sewage system - that is, the siphon that you have at every drain point in your home comes out of it. At the same time, the aeration of the domestic sewage system must be efficient. If the washing machine stinks anyway, it's time to clean it. First, do a few washes at 90 degrees empty. Instead of washing powder, put 2 or 3 dishwasher tablets into the drum.

Excuse me
I accelerated a little, I mistook the balls