Originally Posted by topflightpro

Typically, when climbing seated, sliding back on the saddle is more beneficial to sliding forward. As you move back, you better engage your glutes and hamstrings. Thus, you are bringing in muscles that aren't used as much when riding on the flats. It helps save your quads for once you are done climbing.

Sliding forward, or being on the rivet, generally allows you to gain more power while maintaining a low profile. But aerodynamics are less of a concern when climbing.