Double action
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Double action (also two-stage) is a provisional term for a switch with two sets of contacts with independent actuation staggered by force. The first set of contacts actuates first, and the second set actuates when the switch has been pressed more firmly.
One suggestion is that such switches assist with the activation of an auto-repeat (typematic) mechanism without needing to rely on a timer. Instead, key repeat is based on switch pressure. In the case of the Alps SKCLKB, the first set of contacts actuate at 88 cN, while the second set actuate at 490 cN, allowing the key to function normally when struck (without unintended repeats) but enable the repeat circuitry when held firmly.
Double action switches have so far mostly been found on electric typewriters. The Apple III computer's keyboard has double-action cursor keys where the keys repeat faster when pressed to the bottom.
Various companies provided such switches, including Alps and Cherry.
Wiring
On some typewriters with double-action keys for repeat, all secondary actions are wired to the same input. The keys are told apart by the first action. The repeat-input could also be wired to a Repeat key
Related
Variations of double action press mechanisms are also found in musical claviers, where the time between first and second actuation is used to determine the speed — and strength — of the note. However because a clavier key is a lever on a hinge — where the height is a linear function of distance from the hinge — the mechanism is often instead implemented as two simple momentary switches at two different positions under the lever.
