The rule is simple and every practitioner knows it: whatever is offered, Eshu takes the first portion. The reasoning behind it is a claim about how the invisible world is arranged. The orishas do not receive directly. Speech, praise and sacrifice have to travel, and Eshu is the one who carries them, which means the entire system of exchange between people and orishas depends on a single intermediary who has his own temperament and his own interests. He is not a servant of the other orishas so much as the condition of reaching them at all.
This is why his shrine sits where it does. The characteristic Eshu is a rough head of laterite, sometimes with cowries for eyes, set at the doorway of a compound, at the entrance to a market, or at a fork in the road. Those are places of decision, where one way is taken and another is not, and that is his domain rather than mischief for its own sake. The Ifa verses about him are mostly about obligation: someone is told to make an offering, declines or economises, and the outcome follows. He is the enforcer of the terms. The well-known story in which he walks between two friends wearing a hat that is red on one side and white on the other, so that they quarrel over its colour, is not a prank but a demonstration that two people who saw the same thing can be certain of incompatible accounts.
He is also inseparable from divination. Orunmila, the orisha of wisdom whose corpus the babalawo reads, works alongside Eshu, and it is Eshu who makes the casting speak. A system that claims to deliver true answers needs some account of how the answer gets into the shells, and he is that account.