Summary: This example demonstrates bounce-back, the way that when you press the button, because it is sprung it returns to its original position. Usually the act of having pressed it creates some sort of logical effect, and this example shows how the two can be linked using named events.
This kind of switch you push, but the button does not stay in; instead it bounces back.
This physigram is similar to a toggle switch as both have two states. However, whereas an explicit user action is needed to push the toggle down and up, with the bounce-back the button just pops back out of its own accord. This is depicted by the lightning-like bounce-back transition.
Note also the dashed ‘IN’ state denoting that it is a tension state. You have to actively push the switch to keep it in the ‘IN’ state; as soon as you release the pressure, the button bounces back.
Occasionally this kind of switch is used in a one-to-one exposed state transition, for example if a light is illuminated only while you explicitly hold the button in.
More commonly, the pressing of the button is used as a signal, or event, to cause an action. For example, a keyboard has dozens of bounce-back buttons to type with!
The image given is from a computer, where the button is used to turn the power on and off – one press for on, a second press to turn off. The physigram below denotes this connection to the logical state.
Here there is no one-to-one exposed state mapping. Instead the event of pressing the switch (labelled ‘a’ on the push transition) is used as a trigger for the logical state change.
Because there is no exposed state, devices of this sort often include an explicit indication of the logical state. Indeed in this case there was a small power LED just above the switch.
In this case the use of the bounce-back switch seems poor design. It would be far clearer to have a toggle switch, as that would make the power state immediately obvious.
However, there are times when this is the right design choice. For example, the computer might be designed with a ‘software controlled’ power-down to prevent data loss.