There does not seem to be any special message when this skill triggers. When knocking over the Wall of Crates, which normally gives 5 XP in Strength, I was also getting 1 XP in Will. Nothing special happened when I got 2 or 4 XP in Will, so I'm guessing 5 is the threshhold. (The 4 was from using an item rather than a combat, though, so maybe that made a difference.) I was unable to get 6 or more XP at once before running out of energy and resetting, so I don't know whether the gain is always 1 in the off-stat, or if the bonus increases with larger amounts of XP gain.
Body And Spirit / Discussion
This is the discussion related to the wiki page Body And Spirit.