Replication Question: Who Initiates?

As I understand it, if I am doing replication between a pair of servers, I run the replicator on just one and it makes all the two way changes necessary to make the two machines match. But this only happens when some change happens on that machine that runs the replicator.

First, is this correct?

If so, is there a way to have that other machine give the first machine poke, to get it to run a replication?

Thanks,

-kb