Thanks YW for bringing up this subject.
I’d like to hear more about this from the Oilem, please. I personally tie it very loosely, only in order to prevent it from going flying in a super-strong wind. Otherwise it indeed doesn’t serve the purpose of holding the schach down.
Basically, it doesn’t hold the schach down in a “ruach metzuah”, but it does so in a “ruach she’aino metzuah”, and therefore is permissable.
Now, these were kind of my own conclusions. Am I correct?