I have a hardtail, 2X10 gearing. I have had the chain come off 2 times, both times after a rough downhill section. I'm not shifting during the downhill section, and not pedaling much either. When I get to the bottom, the chain is off the inside of the small ring.
Here's a picture of the crank.



Will one of those simple, $10 gizmos keep this from happening? I've never dropped the chain downshifting. I suspect the chain is flopping around pretty good, and it may happen as I pedal backwards 1/2 turn to get my feet where I want them for the next corner or obstacle. Not sure. Any thoughts?

1) shift into the middle (big) ring on descents.
2) is your chain the proper length?
3) also might look at the front derailleur. the limit screw might be set to allow the cage just a little too close to the seat tube, which would let it shift perfectly while allowing the chain to flop off.
4) use a jump-stop.
https://www.gvtc.com/~ngear/index.html
it's how i keep my ghetto 1X8 from jumping off the ring, so i know it will work for you.

That jump-stop was going to be my first try. For $10, can't go too wrong. It seems that it and other products like it advertise helping to prevent the chain from dropping while shifting to the small ring. I wasn't sure if they would work for what I am experiencing as well.

Chain is the right length, I've adjusted the front der. as well as per the SRAM manual.

the jump stop is definitely the one to get. my sirrus has a plastic one, and it is just aggravating.

Make sure you are not subconsciously pedaling backwards.

1/4 as you say might not be an issue, but more than that it's coming off.

Lesson I learned DH'ing without a chain guide.