Picard had rightly guessed that Geordi would try and reattach Data's 500 year-old head to his body. So the only way of conveying a message 500 years into the future was through the head. As it had only just been severed, there may have still been electrical activity in Data's positronic brain. With the metal, Picard could most likely cause brief short-circuits in the brain, and could in this way create a short message that was stored in Data's memory. Upon revival, Data immediately recognized a pattern in the short-circuits (he mentions a 'binary message from the captain'). Leaving the chip inside Data's head was a message by Picard for his crew to show them he was alive and had something urgent to say.