The hexheads replace the hall effect device with two sensors, one that reads engine speed from the crank and another on the reads location from the camshaft on the right head. This lets the BMS-K engine management know which cylinder is approaching TDC on a compression stroke, something that earlier engine management didn't know (and thus fired both [or all four] plugs at the same time; a wasted spark system).
The crank sensor sits between below the alternator pully, behind the alternator drive belt. I'm not sure if it it reads crank speed only or also can read crank position.
The cam sensor sits below the air intake/throttle body on the right cylinder.
I believe both sensors are variable reluctance type. VR are simple, cheaper sensors than hall effect. The downside is that you need to do some digital signal filtering/processing on their output. That's what the BMS-K is for.