Spirit Shackle's ability triggers only when the enchanted creature changes from untapped to tapped. If Spirit Shackle enters attached to an already tapped creature, it won't do anything (until that creature untaps and becomes tapped again, that is).
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.