As a spider ascends a single strand of silk web where does the slack go?

I have seen them ascend lots of times, but I cannot make out where the slack in the line is going…

Are they leaving it beneath them? I suspect not.

I have a suspicion that they are sort of bundling it up and carrying it with them as they go. The bundling effect might be how they ascend in the first place.

My suspicion is based on what I imagine was a small pill of web that I noticed a spider leave in place after it ascended to a certain point and then decided to go back down.