With the turn of a crank, the head-hunter will draw his knife-arm back and then give it a good whack into the neck of his victim. After that the head-hunter's other hand will raise the victim's head up off his neck and lower it back down. This action will repeat as many times as the crank is turned. Great toy for kids!
This is the process I go through before I build anything. I don't start cutting pieces of wood until I'm fairly certain it will work and be worth the time. If you look at the drawings in order you can see the evolution of the idea. I think it's actually doable in the last drawing.
I'm happiest with this version because it's the simplest. The more moving gizmos you get away from, the better off you'll be. Each new gear and cog and axle is one more thing to give you fits. I needed one horizontal rotating motion for the knife arm, and one vertical motion for the lifting arm. This version of the idea does both motions with only 3 doohickies: A crank-powered axle, a triangle cam, and an off-center cam.
FOR THE KNIFE ARM:
2 pins on the main axle will knock a triangular cam back and forth.
FOR THE LIFTING ARM:
An off-center cam will raise and lower a little wheel attached to the head-hunter's elbow.
I am angry at how simple the answer turned out to be. It may not look like that big of a deal, but it took me days and hours to figure it out. Next time I need those movements though, I'll be ready.
If you are interested in this kind of stuff, you ought to get a book called 507 MECHANICAL MOVEMENTS by Henry T. Brown.
Here's an online copy of the book, so if you're a cheapskate you don't even have to buy it!
Any motion you need, this book will show you how to make it happen.