You will find it throughout 61.129, the aeronautical experience requirements for the commercial certificate. It was originally only there for the multi rating but the October 2009 revision added it for all the other commercial ratings.

If you go a little beyond the reg and into the 2009 Final Rule explanatory material, it's not really an instructor sitting there like a bump on a long. One of the goals is to use the time to improve CRM skills.

But as others indicated, "performing the duties of PIC," at least in the context of commercial training, is not one of the 61.51 "boxes" that allow logging PIC.