The option to enable/disable balancing should be included as a section on this page:

http://docs.mongodb.org/manual/tutorial/manage-sharded-cluster-balancer/

Introduced in 2.2 from SERVER-4621, it is possible to turn on/off balancing for particular collections (which can be useful for troubleshooting, managing migrations, preventing migrations on particularly busy collections etc.)

We document the shell helpers here:

Enable: sh.enableBalancing()
Disable: sh.disableBalancing()

But, we do not explain the feature as part of the balancing management discussion, or really anywhere that I could find.