It was "co-directed" by his long time editor. A lot of footage (supposedly 40% of the final film) was shot during the original Grindhouse trailer. Obviously nothing of Robert DeNiro, Michelle Rodrigeuz (no relation), Jessica Alba, or Lindsay Lohan because none of them were cast in the movie then. Even the sniper who shoots back was the same, but most of the rest was from the trailer.
There is little to no information on who directed what or how this worked. RR is getting all of the credit though, as that's just how marketing works (not necessarily Robert's fault). With PREDATORS and MACHETE, it seems RR is becoming more of a Producer and not a director.
The reasons are this - His main source of funding for the majority of his movies was the Weinsteins, and they are in deep financial trouble, so the only way for him to keep the doors open is to produce for the big studios. RR dropped out of the DGA (union) so big studios have a problem hiring him as a director, but not as producer.
