Lowering a pop-up TC....is there some trick to this?

Hmm, this is the "Heco" lift system, right?

I can stop mine at any point no problem. My camper has about the biggest, heaviest roof ever put on a popup that ever used that system.

Sounds like maybe yours is worn and all loosey-goosey from a lot of use?

Replacing the lift is probably not practical, so you need to devise some way to hold or block the handle from turning while it is unsupervised. Find an anchor point and use a length of rope looped over the handle, maybe?