It was never really clear to me what that was supposed to be. That is, until I watched this.
I'd heard that in the development of the Keen universe, Tom Hall took some inspiration from the old 1953 short film "Duck Dodgers in the 24th and a Half Century". So I watched it. You know, for nerd research.
I was not disappointed.Check out this little segment, starting just after the 5 minute mark.
https://www.dailymotion.com/video/x1ym9b7?start=314
The world makes so much more sense now.
I've previously tried to rationalise everything in the Keen games to fit into the "lore". To absurd levels sometimes. Including the whole "extra lives" situation.
To that end, what seems to be going on with that is those "cartoon deaths" where, although being horribly injured, you're still perfectly fine. I guess Keen just gets whacked right back to the start of the level or something.
Commander Keen has always seemed so cartoony, it just fits so well. My best theory on this before was that Keen had his own android dummies or something, but this works a lot better.It all makes sense. I am happy.
Anyway, it's a nice theory and I think it has credibility. For some reason I never made the connection with this type of cartoony 'damage' even if at the same time I did feel the sprite was sort of cartoony and exaggerated (like the Galaxy death sprite with the bulging eyes). Now that I try to remember, probably something similar happend to Wile E. Coyote in those cartoons -- being hit by a mallet or something?
I too thought he was just reduced to a head and feet, but I like this theory more (in fact, I think it's very likely). When I was a kid, I thought he turned into a puppy head with bat wings. 
