Combinator help - Detect if belt items are moving

I can't quite wrap my head around how to do this, hoping someone might be able to help me.

So let's say I have a belt fully compressed. How can I detect if the belt is moving or backed up?

Essentially what I'm looking to do is turn off a section of my production if the belt is backed up, and turn it on again automatically when it's needed.

I've tried using a belt connected with "hold" and I can turn it on when there's less than 8 items on the belt, but that doesn't cater for being able to fully compress the belt.

"Pulse" seems to be what I need, but how do I detect if it's pulsing regularly and only turn off when it stops pulsing for a while?