The "pillow" mechanism is a secondary set of springs on the back (outer) side of the clutch. When the clutch spins above a certain RPM, the spring tension on these springs is overcome (just like the arm springs) allowing a secondary mechanism called a "pillow" to rotate into place and essentially LOCK the clutch arms to the bell.
As long as your rpms are high enough and your springs tuned correctly (pillow springs ALWAYS higher tension than the arm springs...) the HiT clutch should experience zero slippage under load. It's a very nice feature and works extremely well if you're set up correctly.
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- Mike
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