Spike works as a conduit or transceiver, like with a telegraph you send the message along a wire (this is the magic spell/dragonfire) from a signal box to another signal box. Albeit Celestia could teach Twilight the spell but it is a cool mechanic that is in the show as it shows off Spike’s awesome fire. Happy holidays to all and to all a good-night Mare.
From a tortured soul
Poor Spike. Should’ve just gone to get them yourself, buddy. The walk would’ve hurt less.