In the following documentation:
http://docs.mongodb.org/manual/core/sharding-chunk-migration/#jumbo-chunks

we have:

By default, each document operation during chunk migration propagates to at least one secondary before the balancer proceeds with the next document, which is equivalent to a write concern of { w: 1 }. You can set the writeConcern option on the balancer configuration to set different write concern semantics.

I believe it should be { w:2 }.

This page seems to have it right: http://docs.mongodb.org/manual/tutorial/configure-sharded-cluster-balancer/