Travel refers to the vertical motion of a key.
The total travel of a switch is the distance between the key's resting position, and its position when fully depressed, i.e. how far down you can press the key. In typical desktop keyboards, the total travel is between 3 and 4 mm; keyboards with total travel in this range or above are referred to as "full-travel". "Full-travel" is not formally defined, however; RAFI define "full-travel" as being 2.5 mm or more, which is the total travel of the smaller switches in the RAFI full-travel key switch family.
In most switch types except standard rubber dome over membrane keyboards, a switch will actuate before it is bottomed out. The terms "pretravel" and "operating travel" refer to how far down the key must be pressed for it to actuate. Pretravel is typically around of half travel. Once the pretravel has been reached, the operator is free to let go of the key without needing to press it all the way down.
Overtravel refers to the travel of the switch from the point of actuation to full travel. Depending on the switch technology, and the manufacturer's perspective, there may also be a switching region in between the pretravel range and overtravel range. The term "overtravel" indicates that there is no need to press the switch further: you are exceeding the switch's operating travel. Whether or not the switch will bottom out will depend on the switch characteristics and on the operator.
In some switches (e.g. Marquardt Butterfly the snap action motion of the contacts causes the current to switch from zero to full current instantaneously. In other types (as measured with ITT ETL18) once the contacts meet, the resistance of the switch is proportional to travel. The period in which the switch gradually starts to register is the switching region and this region separates the pretravel from the overtravel.
Various techniques are available to provide an overtravel region. For example, foam and foil switches actuate when the foil disc reaches the PCB; above the disc is a foam "overtravel pad" (hence "foam and foil"), that allows the switch to keep moving even though the disc has reached the PCB. The foam compresses, allowing the key to be pressed further after the foil disc reaches the PCB.
The bridge contact system in RAFI RS 76 M (illuminated) also permits the slider to continue moving beyond the point that the contacts close. The movable contact strip is supported from above by spring, so that the slider can continue moving after movable contact strip reaches the stationary contacts.
Stacked spring switches include an overtravel region by way of triggering a second spring, or a dome, to snap into place under spring pressure before the slider has come to a stop.
Actuation and reset points
The actuation point in a keyboard switch is the point at which the switch registers a key press. Similarly, the reset point is when the key ceases to register the key press. In some descriptions, the actuation point may be referred to as the "make" or "operating" point. The reset point may also be referred to as the "break" point.
In many switch designs, the actuation and reset point do not occur at the same place in the key travel. Instead, the reset point occurs higher up in travel than the actuation point. The gap between these points is referred to as the hysteresis, or lost motion. Hysteresis prevents some repeated keystrokes from occurring, but not all.
The hysteresis of the Cherry MX Blue switch is quite pronounced due to its design, and this means it becomes harder to rapidly press a key multiple times in a row ('double tap'). This makes it less suitable for playing certain types of games.
The term "Speed", apparently introduced by Cherry, describe switches that actuate faster. Such switches simply have reduced pretravel such that actuation occurs sooner within the keystroke, and typically the total travel is also reduced. Examples include Cherry MX Speed Silver and Kaihua PG1511 series. Speed switches appear to be intended for gamers.