If someone was walking across the yard it would save every frame. The movement of the sun would move shadows enough to trigger a new image every few minutes. A bug flying past was small enough that it wouldn't trigger anything. The result was you could get a short video of everything interesting that happened through the day: shadows of trees sliding over the ground, every frame of the car pulling out of the driveway, shadows sliding over the ground some more, cat walks across the yard then lays down, shadows pan around more while the cat sits still, cat gets up and walks away, shadows pan around until the delivery guy comes...
It was an incredibly low-CPU way to see everything that happened without missing anything, and without having to fine-tune the motion detection very much. You just mask out any areas with constant motion, then adjust the slider for how much change triggered the next frame, which would let you adjust how fast the timelapse would go during the boring parts.
I've always wondered why the technique never became widespread.
Essentially, the same concept, just need enough of a buffer to allow for the pre-roll which wouldn't be a lot at the lower bitrate IP data coming from the cameras
Unfortunately consumer devices are almost always cloud-based, where storage but especially upstream bandwidth are much more costly considerations, so recording only on detection has become the norm in the consumer world.
External triggers are also an important feature in commercial systems that a lot of open source projects miss---but Frigate isn't guilty of this one, it can receive triggers by MQTT, which is the same thing I do right now with Blue Iris. That's the big thing that has me optimistic about Frigate going forward. Because motion and object detection are so inconsistent, triggering VMS events based on access control systems and intrusion sensors is often a much more reliable (and even easier to maintain) approach.