When a hiker going uphill encounters a hiker going downhill, the hiker going downhill yields to the hiker going uphill.
When a trail runner going uphill encounters a trail runner going downhill, the runner going uphill yields to the runner going downhill.
When a trail runner going uphill encounters a hiker going downhill, they yield to each other and just stand there looking like idiots.
When a trail runner going downhill encounters a hiker going uphill, neither yield. They collide, limp home, and whine on their respective Internet discussion fora about how rude the other population of trail users are.