If an effect would set your life total to a specific number that's higher than your current life total, that effect causes you to gain life equal to the difference. The Wind Crystal will then double the amount of life that effect would cause you to gain. For example, if you have 3 life and an effect says that your life total "becomes 10," you would gain 7 life which The Wind Crystal doubles to 14, so your life total will actually become 17.