It’s just a low pass filter applied to the motor output signals. Essentially smoothing out any rapidly changing signals.
What it does very well: creates a very clean “off throttle” response even with an imperfect tune. In other words, you punch out and chop throttle to 0… The quad idles down very quickly and cleanly even if some noise made it through the pid calculation.
What it does poorly: sometimes a rapidly changing motor signal is exactly what we want … Like in propwash or at the end of a flip or roll. Aggressive motor output filtering can make these moments have a little more bobble. Similar to the bobble you would see with one lazy brushed motor. So much of what we do is about increasing motor response… Torque boost, d term setpoint weights, throttle boost - a low pass filter on outputs is kind of against the grain. While low frequency changes are only delayed a tiny amount… Higher frequency changes are damped out and any immediate change that we want to have an abrupt edge becomes rou1nded off.
My thoughts, it’s a bandaid. Which is great when you need one…but I’d rather not need it in the first place. My brushless does not need it at all, and runs clean with one 1st order gyro pass and one 1st order D pass. But pids are much lower so noise is amplified much less. I’ve never been able to avoid it before on brushed because of the much higher pids. I did get close before using a second order gyro filter on my whoop instead of a first order …which is pretty similar to the cascading 1st order filters we are doing here… But I lacked the flexibility to really dial it in with 2 adjustable cuts which gives an opportunity to save a little latency and push P a little higher.
Hope a clear answer is in there somewhere.