Actuation refers to the point when a keyswitch changes from being open to being closed, i.e. the point when the keyboard will register a keystroke. The actuation point can also be referred to as the "make" point. In quality switches, the actuation point occurs before full travel, i.e. before the switch is bottomed out; typically actuation will occur at half travel, but this varies betweens switches. Typical discrete switches have 3.5–4 mm of travel, with actuation around 2 mm. Membrane rubber dome switches only actuate when fully depressed.
Release refers to when the switch returns to its default closed state, i.e. the keyboard detects that the key has been released. This can also be referred to as the "break" point.
In some switches, such as ITT ETL18, there is a portion of the travel where resistance is proportional to travel. Testing with an analogue meter, pressing the slider slowly causes the needle to move across the dial from full resistance to near-zero resistance as the slider descends; if the slider is held at a position within this region, the resistance will also remain fixed at an intermediate level.
Some manufacturers separate the travel into pretravel, overtravel and switching region. Examples of switches with manufacturer-specified switching regions include Alps SKCL/SKCM series and Clare/Pendar low profile and high profile reed. In the case of Clare/Pendar, the switching region is defined implicitly by the gap between the pretravel and overtravel regions, while the 1994 Alps catalogue visually depicted the switching region for SKCL/SKCM.
Testing with SMK J-M0404, Cherry MX and Alps SKCL switches shows that switching is effectively instantaneous (with a tiny amount of hysteresis), and only with J-M0404 was it just about possible to hold the slider at an intermediate resistance. As such, the definition of "switching region" is not clear.