It has a cool down to switch targets and then yiu build a stack every 60 seconds with a 90 second CD to switch. That means if you switch targets you lose all the damage resistance and the anti tenacity.

If you out it on your lane opponent when you get the item at like 8 minutes into the game, then you switch it to the fed enemy carry at 15 minutes, you won't have it fully stacked until 45 minutes (30 stacks 60 seconds per stack) and then you wouldn't be able to switch back to your lane opponent for holding sidelane pressure unless you can wait at least 10 minutes to get some effect back.

The description needs to be explained better because lots of people don't seem to get what exactly the item does.