Although it never made it into production, Bell developed a crew escape system for the two blade AH-1 Cobra. The most unique feature was the main rotor blade separation system. The system pyrotechnically severed only one blade at the cuff, and relied on the resulting rotor imbalance to rip the mast off the gearbox.
CTR: very interesting, I've never heard of this particular 'feature'. I can imagine the massive imbalance would rip the mast off the gearbox (or the entire gearbox off the airframe) in fairly short order, but what would it do to the crew's necks in the tenth or half second it took? I'm sure some smart Bell engineers considered this, perhaps why it never went into production.