When non-nil, shifted motion keys activate the mark momentarily.
While the mark is activated in this way, any shift-translated point
motion key extends the region, and if Transient Mark mode was off, it
is temporarily turned on. Furthermore, the mark will be deactivated
by any subsequent point motion key that was not shift-translated, or
by any action that normally deactivates the mark in Transient Mark mode.
See `this-command-keys-shift-translated' for the meaning of
shift-translation.