The sad story is, it had an indicator over your hand pointer. I removed after a playtest thinking that discovering would be more fun, as the HUD with the birds already shows the progress. But guess what? It looks like many players are not discovering it naturally T^T lesson learned.
Maybe it is actually more interesting to discover by yourself, but you run the risk of letting many players without more necessary info. In an ideal project, with more time, I could show the icon after understanding that the player is not reviving the units. Maybe after they lose a level without trying to shake any units or something like that.