Magno-grab remote

A magno-grab remote was, as the name implied, a remote control for a magno-grab. Gregor Van Baalen used one to generate the magnetic hobble-field that crippled the TARDIS. The Eleventh Doctor then stole it off the salvage merchant, and later sent it through a crack in time to his earlier self, whereupon Clara picked it up and embossed a message the future Doctor had carved into it onto her hand. This message, "big friendly button", subsequently became the clue that helped the Doctor understand how to use the crack in time to save the TARDIS. Since Clara had just asked the Doctor whether there was some "big friendly button" that he could push to make the emergency stop, this message indicated that the "big friendly button" was the one on the remote.

Using Gregor's remote onboard the TARDIS to counteract the signal that had been sent from Gregor's ship allowed the Doctor to turn off the hobble-field, and thus avert the entire chain of events that damaged the TARDIS. (TV: Journey to the Centre of the TARDIS)