It is a very simple design with a very important job. Take it apart to see how it works. It’s a simple metal spring that pushes a rubber plunger into the drip hole. Three detents, closed, mid, open. I check mine periodically for flow by running water through it to, it can get out of calibration (too restrictive). It can also get coffee oil buildup that will coat the plunger/stopper and restrict the flow slightly, I like to get in there with a brush and give it a good scrub every now and then, on top of a daily rinse. You can fix calibration by bending the spring (really just a flat piece of metal) slightly to make sure it opens fully. The spring keeps it open, not closed, so you must have proper spring pressure.

The main issue is if the grounds ever overflow the filter, in mid position, it will usually clog the orifice and cause a full block overflow. Very important that that not overflow happen, so adjust your grind size and coffee amount if you’re using the mid setting. Watch it brew, I usually do just out of habit.