They don't actually fire those off their knees, do they?
Nope. They are a small mortar which fired hand grenades with a small propellant charge attached to the bottom. They have a curved baseplate which is meant to be placed on the ground, but looks like it would fit against the curve of the thigh of a kneenimg man. Many US soldiers assumed this was the correct firing position, supposedly resulting in many broken legs when they tried captured mortars. I’ve seen one in a museum, and they are pretty simple.
“Thanks. I see the “knee mortar” legend lives on.
They don’t actually fire those off their knees, do they?”
Not more than once. Just a mistaken nickname. It gets you a broken thigh. The base plate is meant to be placed on the ground.