The Screw Stack is named after the two defining construction points of these pickups. The poles are unlike typical Strat single coils - they're powered by dual mini ceramic bar magnets charging the adjustable pole pieces. Typical Strat single coils use charged alnico rods.
The stack part comes from the 'stacked' coils. There's two coils, just like any other humbucker. But they don't sound like the pickups you normally associate with 'humbucker'. The two coils work together to remove the background 50/60hz hum that usually comes with single coils.
The bar magnets, steel poles and stacked coils give these pickups a warm, sweet tone. The noiseless operation makes them a perfect pick for matching with humbuckers in HSS or HSH pickup sets, they also make excellent bridge pickups where typical Strat single coils can be too bright and tinny.