When you press the switch to its full depth. Mechanical switches actually actuate before you bottom out.
The "standard" mechanical keyboard switch. Originally created by the German company "Cherry" in the 1980s. They come in different colors which corresponds to the different kinds of switches. e.g. Red - Linear, Brown - Tactile, Blue - Clicky.
Image from https://github.com/ai03-2725/Unified-Daughterboard
A daughterboard (or daughter board, daughter card) is a circuit board that plugs into the PCB. There is an attempt by leading designers in the mechanical keyboard community to standardize the USB daughterboards. It was originally envisioned by: ai03, Wilba, Hineybush, Xelus, Xondat, aeryxz and Maximillian.
GMK is a german company that designs luxury keycaps for mechanical keyboards.