The tough guy takes out a grenade and pulls the pin with his teeth, then tosses it to kill a bunch of mooks. It's a common trope in action flicks and war films, so much so this troper has lost count of how many times he's seen it and was more than a little surprised to not find a page for it already.
In reality, grenade pins are designed to be hard to pull so they are never pulled accidentally. Attempting to pull them with your teeth will likely result in some painful dental damage.
This troper's most recently witnessed example was close to the end of Inception.