Offset is wrapping keys and setting keys at the start and end of the animation. You could do the same manually, but Offset is more convenient. Doing this adds 1 new key. The Offset button turns orange to indicate that the original keys are remembered. You can move keys multiple times and Spine will use the original keys then apply the offset so only 1 new key is created.
When you deselect or the selection changes to different items, the original keys are forgotten and the button is no longer orange. If you then use Offset again, it will create 1 new key.
Offsetting cannot be done without creating the 1 new key. You can avoid creating more than 1 key by doing all your offsetting at once, without changing the selection.
