You must pay its mana cost (or, if another effect allows, an alternative cost)
Both cheat equip costs and draw you cards, and both will generate consistent value for you
Still, if you find the right time to do it, Bahamut, Warden of Light does a good job of ending the game for you
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As another example, if my opening hand has a one-drop, a two-drop, a three-drop and four different four-drops, then I will usually bottom the worst three four-drops in the hope of drawing into a powerful five or six-drop, while keeping the cards that will ensure a perfect 1-2-3-4 curve-out
You do this without revealing the cards, and you then draw cards until there are seven in your hand once again