It's supposed to give you the same amount of cards that are in your hand. It's basically another mulligan you can use at any time or with "bottom of the deck" shenanigans. What you likely saw was just an animation glitch or if you had Brann Bronzebeard it would replace your hand once, and then a second time.
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"Speculation is foolish when the tools of certainty are available." —Cinna, Vedalken Consul