It's a spacer, It goes under the very top screw hole of the main board, and aligns up against edge of the top housing, with the longest edge of the spacer near the power button. In the tear down guide refer to step 23 or 27 of the comments, I posted more info there.

I may have some useful info for those that aren't having any luck with this process. Prior to trying this I checked the voltage of the battery which was 3.7 volts, fully charged. After following this process without success I found after each spark the battery voltage had dropped to a low 1.0 volt, it would then require me to recharge the battery before I was able to get additional sparks. Is it normal for the volts to drop so low after a spark ???