Incremental linking means the general idea of reusing previous linking results, only patching it up with respect to changed obj files. So it's about reducing link times, not compile times. This has various consequences for executable size etc. so not something you'd want to do for release builds I think...

Here's the documentation for VC's incremental linking option: http://msdn.microsoft.com/en-us/library/4khtbfyf(VS.80).aspx