"Keen," or "fast" ice means that the rocks are sliding down the ice very easy, hence players don't need to throw the rocks very hard. The opposite of keen ice is "heavy" or "slow" ice.
"Swingy" ice means that the rocks are curling alot as they travel down the ice (say, 5 or 6 feet instead of the usual 3 or so feet). The opposite of swingy is "straight" ice.
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