You left the double track 5 miles back.  You're still 10 from the car and it's getting dark.  Were those lion tracks back there?  You pick-up the pace but in your haste you get sloppy, tear off your derailluer on a root.  Chain is balled up around the cassette and looks like it's been through a meat grinder.  You're at 8'000 ft - it's getting cold. 
 
Better work fast.
 
Unscrew the end cap from your hande bars and yank out your tool (Gnarly Trails Miracle Chain Fix-it Tool).  First, break the chain at the (SRAM type) quick link - should only take a minute or two.  Unwrap the chain from the cassette.  Then identify the twisted links; put one lever on each side of the twisted link(s) and gently twist in opposing directions.  Repeat for the other twisted links.  At least now the chain will stay on the rings.
 
You may not make it back before dark, but at least you're not walking.
 
This svelte little tool does two things really well.  It breaks a SRAM type quick link (for most modern chains) and it can unbend twisted links.  It weighs next to nothing so it won't drag you down and it fits inside your handle bar so it doesn't take up room in your kit.
 
Yes, there are other bicycle chain breakers out there but they're bulky and heavy and made of nylon and plastic and who needs that in the essential bike tools you carry when you ride?  Besides, none of the bike chain tools can unbend a chain like the Gnarly chain fix-it tool can.