It is the spark plug insulator. They are using the ceramic from it to break the glass. I remember hearing about this in the 90s with thieves carrying insulators on strings in their pockets.

https://www.carparts.com/blog/can-a-spark-plug-break-a-windo...

note from the article: "Keeping Spark Plug Shards Might Be Illegal in Your State" so keep that in mind