The Core Drill is the key that’s used to turn on Lagann. A small, key-like drill that Simon finds during one of his digs and is subsequently almost always seen in his possession or in the keyhole powering Lagann. The Core Drill is shown to power Lagann. The Core Drill can also sometimes be seen glowing from within with spiral power.
The Core Drill is also not the only one of its type, as Lordgenome knows of its existence, and by the fact that the rest of Team Dai-Gurren is able to obtain one for each member to power a Lagann during the final two episodes of the series.so
Simon finds the Core Drill in the dirt while digging a tunnel in Jeeha Village. He immediately stops his work and leaves the tunnels, and bumps into Kamina, who reassures him that "... drills are your soul, Simon."
Later, after bringing Kamina and Yoko to the unearthed Lagann, Simon figures out that the Core Drill is some sort of power source for Lagann, and uses it to start the mech. From then on, it is almost always seen powering Lagann or around Simon's neck. The Core drill is then passed on to Gimmy Adai after the death of Nia and Simon's chosen exile.
Apparently, the Core Drill acts as a conduit for Spiral Power and is not limited to only powering Lagann. This is seen when Simon stabs Lordgenome in the chest with it and turns it, resulting in a spectacular blast of Spiral Power that rips a hole in Lordgenome's chest.