The "Lightning Tree" in the arena was used to escape. At midnight and noon lightning hits this tree as one of the trap set in the arena. Beetee had the idea to wrap some wire around the tree and put the other end of the wire in the water. He tells Katniss that this is how he plans to kill the Careers that have their camp set up on the beach. The plan is that when the wave hit the beach the beach will still be wet at noon when the lightning hits the tree. When the lightning hit the tree the electricity will travel through the wire in to the water. This will electrocute the tributes on the wet beach. But the actual plan was to overload the force field so Haymitch could come in with the hovercraft and take them out of the arena.