OK - be warned - I know little about this.
The rotator caps (used on exhaust valves on some stovebolt engines) were also used with longer springs (and, the caps were thicker than normal caps). This resulted in greater valve spring "pressure". In engines that used the rotator caps, the valve spring "pressure" was significantly greater. The higher spring pressure was used due to the higher lift of the 261 cam (a cam used in 1954/55 Corvette 235s and in all 235s in later years).
This higher spring pressure presumably resulted in better heat transfer (cooling) at the contact surface, and I guess, lower possibility of "valve float" (especially at high RPM). On some engines (maybe not stovebolts?), valve float might result in a valve hitting a piston.
On stovebolts, different composition valves were used with the rotators (Silchrome XCR).
Exhaust valve rotators were first used on heavy-duty stovebolt engines (in 1954, I think); but, by the end of the 235 "run", I think they were used on all 235s & 261s.
