It’s clearly stated here that Ctrl + [ should jump the cursor to the start of the selection as displayed on Start and end of selection panel.
However, this is not working for me: simultaneously pressing both “Ctrl” and “[” does nothing.
That shortcut is part of the “Full” set of shortcuts.
To enable the Full set go to:
“Edit menu > Preferences > Keyboard”
Then click on the “Defaults” button at the bottom and select “Full”.
See also: https://manual.audacityteam.org/man/keyboard_preferences.html
I’ll add a note about this to the next version of the manual.
The note will be useful for new users.