Sometimes, despite your best efforts, arguments won't go your way. You've tried all the tricks that you know but your opponent is just too agile to be taken down. Defeat looms on the horizon.
At times like these, there's only one thing for it: throwing out a red herring. A red herring is a side-issue, a rabbit trail, a distraction.
Throwing out a red herring is like rebooting your computer when everything's gone wrong. Programs are crashing all over the place, your mouse cursor won't move, you can't cancel all the error messages that have popped up onto your screen, so you reboot and start again from scratch.
Throw out a red herring, change the focus of the discussion, and no matter how badly you were doing before it doesn't matter, because on this new issue everything's to play for.
The key to the red herring is not to make it too obvious. You don't want it to seem as though you're conceding defeat by abandoning the first topic. You must therefore insist that the side-issue that you're raising is fundamental, and that it won't be possible to get anywhere with the main debate until you've cleared it up.
If your opponent denies this, and tries to drag you back to the matter at hand, then accuse them of being evasive.