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Only Iron Ball is special really; if you're holding it before getting hit by Smack Down or Thousand Arrows you can't become grounded and will always take neutral Ground damage unless Gravity or Ingrain is also affecting you, but if you become grounded by the moves and then get an Iron Ball, you take non-neutral Ground damage all the time until you switch.
Of course, this stuff only applies to Flying types. Your guess is as good as mine what actually is going on.
Only Iron Ball is special really; if you're holding it before getting hit by Smack Down or Thousand Arrows you can't become grounded and will always take neutral Ground damage unless Gravity or Ingrain is also affecting you, but if you become grounded by the moves and then get an Iron Ball, you take non-neutral Ground damage all the time until you switch.
Of course, this stuff only applies to Flying types. Your guess is as good as mine what actually is going on.