I'm a bit confused based on the description of precheck here. I've used it only once in LAS (through AA), and there was a separate precheck line in the elite area that I could self-select to use. In other words, rather than waiting in line for TSA to scan my boarding pass and tell me I qualified, I just got into the precheck "line" - where there was nobody waiting. I imagine if I didn't get selected, I would have been shifted over with all the other elite/F passengers, but I still would have skipped the wait just to have my boarding pass scanned.