Return to Ravnica new mechanics Explained (Part 5 of 5)

Detain
When you detain an opposing permanent, until your next turn, that permanent's activated abilities can't be activated and (if it's a creature) it can't attack or block. Activated abilities are the ones that have a colon (:) in them, such as "
: Add one mana of any color to your mana pool" or "Equip (: Attach to target creature you control. Equip only as a sorcery.)"

Detain is best in decks which rely on fast small creatures attacking every turn. This is achieved by not allowing your opponent's creatures to interact you or your creatures. Along with creatures you can use spells such as Sleep or Feeling of the Dread, to open the way for your creatures to attack without opposition.