Can somebody explain "auto-climb" and its sub-settings to me?

[new Fenix purchaser]. The setting "auto-climb" says (manual): "Enables the device to detect elevation changes automatically using the built-in altimeter." For my first walk, it was set to "off" (default apparently) but the walk still gave me stats for total ascent, descent for the walk. So, what does this setting do, exactly? It does not appear to completely turn off the altimeter when set to off, or so it appeared to me.

Also, "auto-climb" has two sub-settings, vertical speed and mode switch.
  • Vertical speed is defined: "to set the rate of ascent over time". What rate are you setting? What effect does setting this rate have?
  • Mode-switch is defined: "to set how quickly the device changes modes". What modes? What does this setting mean?

I do not find the manual all that well written! Thanks for your help.