Split horizon, in EIGRP, affects the sending of update and query type packets.
Update packets contain route change information (sent reliably, needs ACK).
Query packets are sent when a router does not have a feasible successor which is a next hop router for a backup path (sent reliably, needs ACK).
Split Horizon is a way to prevent (minimize) routing loops. The logic is that if a router uses interface A to get to a destination, there is no reason to advertise that route out intA.
For example, if network 10.1.1.0/24's next hop router is located out fa0/0, then the router will not send update packets containing 10.1.1.0/24 out of fa0/0, only out fa0/1, fa0/2, etc.
If you think about it, if router A is your next hop to a network, why would you need to tell router A how to get to itself?
Split horizon is on by default on all interfaces.
If a router changes the interface it uses to get to a destination, it momentarily turns off split horizon and commits route poisoning. Route poisoning is when the router, in order to stop other routers from using its old route/cost that it just changed, floods out route unreachable packets. This causes other routers to flush that old route out of their routing tables so that the new route can now be successfully added.
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