Imagine your replicator database (default name is _replicator) has the two following documents that represent pull replications from servers A and B:

{
    "_id": "rep_from_A",
    "source":  "http://aserver.com:5984/foo",
    "target":  "foo_a",
    "continuous":  true,
    "_replication_id":  "c0ebe9256695ff083347cbf95f93e280",
    "_replication_state":  "triggered",
    "_replication_state_time":  1297971311
}
{
    "_id": "rep_from_B",
    "source":  "http://bserver.com:5984/foo",
    "target":  "foo_b",
    "continuous":  true,
    "_replication_id":  "231bb3cf9d48314eaa8d48a9170570d1",
    "_replication_state":  "triggered",
    "_replication_state_time":  1297974122
}

Now without stopping and restarting CouchDB, you change the name of the replicator database to another_replicator_db:

$ curl -X PUT http://localhost:5984/_config/replicator/db -d '"another_replicator_db"'
"_replicator"

As soon as this is done, both pull replications defined before, are stopped. This is explicitly mentioned in CouchDB's log:

[Fri, 11 Mar 2011 07:44:20 GMT] [info] [<0.104.0>] Stopping all ongoing replications because the replicator database was deleted or changed
[Fri, 11 Mar 2011 07:44:20 GMT] [info] [<0.127.0>] 127.0.0.1 - - PUT /_config/replicator/db 200

Imagine now you add a replication document to the new replicator database named another_replicator_db:

{
    "_id": "rep_from_X",
    "source":  "http://xserver.com:5984/foo",
    "target":  "foo_x",
    "continuous":  true
}

From now own you have a single replication going on in your system: a pull replication pulling from server X. Now you change back the replicator database to the original one _replicator:

$ curl -X PUT http://localhost:5984/_config/replicator/db -d '"_replicator"'
"another_replicator_db"

Immediately after this operation, the replication pulling from server X will be stopped and the replications defined in the _replicator database (pulling from servers A and B) will be resumed.

Changing again the replicator database to another_replicator_db will stop the pull replications pulling from servers A and B, and resume the pull replication pulling from server X.