The write concern of secondaryThrottle can be fine tuned (SERVER-14041), and is applied at pre-commit (as well as the normal w:majority WC) in case it's stronger than w:majority (or uses tags to target certain nodes) (SERVER-15591). However, a valid use case is a stronger/more targeted WC at pre-commit, but without slowing down the whole migration (as using secondaryThrottle would).

This request is to allow a custom pre-commit write concern to be configured and applied (in addition to w:majority), i.e. similar to SERVER-14041/SERVER-15591 except only at the end of the migration, not the whole way through (which is what secondaryThrottle is). (wtimeout should not be supported at pre-commit, since it would always be pointlessly unsafe.)