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I'm not an expert - but your SteemPower for the day is divided by the number of likes. So if you have a $1000 worth of value to give, and only make one like, that post gets the entire $1000. If you make 5 likes, then each post gets $200. It keeps going down the more you like. Undoing likes resets what your previous likes have got. And if you "rest" for 24 hours, the power restores itself.

If you vote less than 20 times a day your STEEM power gets divided by 20. So if you vote 10 times a day each vote will still be worth 1/20 * STEEM power. For 21 or more it will be 1 / amount of votes made today.

I think it means that any new votes cast on that post will be calculated as if you had never voted before. For example, if you had cast the first vote on a piece of content that had been voted up, by resetting, you forfeit any associated curation rewards. If you then voted on that same post again, you would receive a smaller, if any, curation reward.