User blog comment:Fistful of Force/Cast heals early, not late./@comment-195.69.193.12-20120612123600/@comment-75.145.172.201-20120613054221

Yea, I don't mean to spam it in case someone attacks you, but rather to cast shield immediately when you see the damage incoming (attack animation, projectile, etc.) That's also why it's important to stay close to heal/ shield targets: So you can shield/heal them in time.