In each channelstrip, you will find a list of the plugins applied to that channel. Each plugin has a green button which indicates it's on/off ( or "enable/bypass") state.
Many plugin formats provide a mechanism for the plugin to cleanly toggle the plugin in&out, click-free. Only the plugin developer knows how their plugin should be cleanly bypassed. Bypassed plugins may still take up some DSP power, although most plugins will use less power when bypassed.
Plugins that do not provide this feature, will be hard-bypassed when you disable the plugin in Mixbus. In this case the processor will use '0' cpu but it might cause a click when bypassing/enabling.
Some plugins have a wet/dry knob which can effectively be used as a click-free bypass.