Witch- you're talking about something completely different. A superstall is where a higher-set tailplane at very low speed/full wing stall is 'blanked' by very disturbed airflow off the wing, and thus elevator authority is totally lost. This resulted in one of the early Tridents on Salisbury Plain being lost with the flight test crew, a BAC 1-11 similarly so, possibly the Staines Trident, and no doubt several others, along with a very frightened Notso in a VC10 on a CofA test over Anglia at 17,000'. I didn't want to do it or be there, but I was. We had an AofA meter fitted. It hovered at about 15 degrees with extensive airframe vibration and Captain holding wings level, then it jumps. I was told if it goes past 17 degrees, you're superstalled and you die. Pprune will be forever grateful it stopped at 17, I think I saw 95 knots ASI and Notso swore never again, under any circumstances, should he ever, whilst trying to bring up a family, be involved in such nonsense again, which is why to this day I do not parachute, hang glide, paraglide. Waterskiing and snowboarding has been an exception.
But to get back to the question, the phenomenon of superstalls exists because it does- you cannot ever have a wing that does not stall. It was recognised that someone, somehow, somewhere, would get into one in an aeroplane with a high tail, it's known that pilots are fallible, so stick pushers are there as a last resort. Recognising that all wings eventually stall, you must therefore ensure the last place to stall are the tips because:
1- as BOAC says, you will get a nose up moment which is disastrous and adds to your problems
2- if it is assymmetric for any reason such as sideslip or gust, you will suddenly find yourself upside down.
Examine closely wingtip areas in swept wing jets, even in photographs on airliners.net. You will see the varying design tricks I outlined to delay tip stalling. Even on the 747-400 it is plainly visible. You will always be able to see reduced incidence and/or reduced sweepback.