Which nVersion bit is the "hard fork bit?"
Bit 31 is sometimes referred to as the hard fork bit.

BIP34 put a constraint on version numbers to be 2 or higher. As the block version is a 32-bit signed integer, setting its highest bit results in a negative number, which would violate BIP34.

As a result, any usage of that bit results in a backward incompatible change to the rules - a hard fork - that is obvious to all participants in the system.