and for the curious who didn't what Chernobyl on HBO, other types of reactors have brief criticality increases on rod insertion, which is somewhere between bad, BAD, and OMFGWTFBBQVERYBAD. In the Chernobyl's case, they entered a criticality region known as "prompt criticality" which is where power doubling can happen at prompt neutron rates (e.g. milliseconds or faster). Very very bad. In the case of pressurized water reactors, this isn't what happens. Every moment of control rod descent is reducing criticality.