For information on aftermarket and vintage sliders that can be used with HHKB, see Sliders under Modding.
According to the Deskthority wiki:
The switch consists of a slider in a housing over a rubber dome over a conical coiled spring over a printed circuit board.
The slider contacts the housing (in the case of a stock HHKB, the housings are molded into the top case) and the keycaps are attached to the top of the slider. The bottom of the slider contacts the rubber dome, which provides tactile feedback when typing.
While not technically a mechanical switch, Topre keyboards are often included in the greater discussion surrounding mechanical keyboards. This is partially due to their novel actuation method (until the mid-2010s, no other electrostatic capacitive keyboards were widely produced), their relatively high price (grouping them in with mechanical keyboards as luxury peripherals) and the (limited) availability of replaceable keycap sets.
There are two slider types available on stock Happy Hacking Keyboard models. Many people who modify their HHKBs also make use of slider designs found in Topre Realforce keyboards, but that requires obtaining a separate keyboard from which to harvest sliders and other components.
All OEM HHKB sliders are black.
The standard slider design, found in all HHKB Professional models (excluding the Type-S variants) is more or less exactly the same as the stock Topre Realforce slider design.
The Type-S variants feature OEM silencing rings that sit between the top of the slider's base and the portion of the case that comes into contact with the slider's base. This reduces the sharper, louder reverberation produced upon contact of the underside of the slider housing after the key is pressed when the slider returns to it's non-depressed state.
Type-S sliders also feature a depression in the underside of the slider body (the part that makes contact with the rubber dome). This counteracts the silencing ring and allows the slider to sit in more or less the same position as a non-silenced slider in relation to the housing and the rubber dome. In this way, the housing/top case design can be shared between the standard models and the Type-S models - likely an engineering decision based on economics. It would be significantly more expensive for Topre to produce and maintain separate top-case molds than it would be for slider molds.