"When two vehicles meet on a steep hill, the vehicle traveling up the hill has the right of way. This is because the vehicle traveling uphill may need to maintain momentum, and because it is more difficult and dangerous to back down a steep narrow trail. Common sense should prevail though; if it is easier and there is room for the uphill vehicle to pull over, it wouldn’t make sense to expect the downhill vehicle to back up the hill. Either way backing up is tough. If you are going up a big obstacle like a long, steep rocky climb, it may make sense to send a spotter up on foot to make sure the trail is clear and to warn any on-coming vehicles."
I guarantee that at least half the people I encounter on trails don't know this rule. I had a long argument with a tool from Texas on a very narrow trail here in CO once. He insisted that I needed to back about 1/4 mile DOWN the trail to the nearest pullout, to keep him from having to back about 200 feet up the trail.
The person going UP has the right of way. Don't be that guy.