A latter-day practice — possibly begun by Craig Biggio, but vastly inculturated by the Red Sox — of putting pine tar on the helmet, so if you need some quickly during your at-bat, you can just go to your helmet. I think Biggio initially just gunked up his helmet by constantly adjusting it during his plate appearances while handling his bat, but it became a thing to transfer pine tar in the opposite direction. There's buzz out there that MLB may soon be cracking down on this, as well, but it feels like it's been going on for a while now.