Respin triggers when the spin is a losing spin, AND at least one zero or double zero appears in one of the reels. When re-spin is activated, the reels which aren't zero or double zero respin a single time. If the respin resulted in a payout, the feature ends.
If the respin resulted in another losing spin AND at least one more zero or double zero appear in one respin reel during the first respin, the respin reel which isn't zero or double zero respins for the second time. The resulting payout of the second respin is final, and the feature ends.