This is a knife that once inserted injects supercooled gas into the target, designed to freeze and rupture internal organs. Wait, here, they explain it in cooler language than I can:

This weapon injects a freezing cold ball of compressed gas, approximately the size of a basketball, at 800psi nearly instantly. The effects of this injection will drop many of the world's largest land predators.

They designed this thing to work on land or in the water. So watch as they test this thing against one of the most vicious land predators.

Direct link for the feedreaders.

So what have we learned? That the Wasp Knife is effective against watermelons but only one at a time. If you get cornered by a sarong of watermelons (the technical term for a pack of watermelons) then you're fucked and won't have time to reload. The second watermelon here was obviously sedated, otherwise it would have had the guy's face off before he could get the new canister in place.

Found via Zombiegeddon.