When you "go down" on a male's asshole and cover it in tarter sauce to mask the shitty smell or ass juices. You can use either mayonaise or jelly.
When "going downhill", you must cover it with one of the topping to mask the scent down there, and you must bury your face down in between the male's asscheeks and eat out his asshole like you would a female's vagina. Do lots of sucking and lots of tongue power.
You: Hey bro, I went downhill on Tom yesterday.
Bro 1: Awh, what the fuck bruh! That's disgusting!