Here is more information about the tradition of exploding hammers:
Thanks to someone reading the comments, I have located the person responsible for shooting the video. They have been sent a link in case they are not happy with this use.
The Ideal Gas law (pV=nRT) was first stated by Benot Clapeyron in 1834. Basically, from it we can deduct that if you compress a material quickly, it generates heat because of friction between the gas molecules.
In fact, this principal is what powers diesel engines. Of course, there are no gasses involved in the base explosives, the Ideal Gas law is cited here because it demonstrates the relationship between friction and heat. What triggers the detonation is a combination of shockwave and internal friction - that's why you have to use fireworks with chlorates or perchlorates in them (thanks kiteman).
Instead of a spark plug to ignite the fuel, diesel engines have a glow plug to heat the fuel to a critical temperature. It is the compression from the stroke of the piston itself that causes the final ignition!
In this instructable we will show you how to illustrate this principal while building an exploding sledgehammer. Be careful as this may be illegal and/or very dangerous.
Step 1: Materials
Sledgehammer, fireworks, masking tape. You'll need a sledgehammer of decent size. It must have a business end heavy enough to combust the explosives and a handle
long enough to give you distance from the explosion (for safety).
The fireworks can be anything that contains, mixtures of potassium chloride and sulphur. Many do. Regular gunpowder may work, too. The masking tape can be substituted with other kinds of sticky tape.
Step 2: Taping Things Together
Tape the fireworks on one end (and one end only) of the sledge hammer. Make sure they are bundled tight so that the sledgehammer will deliver all
it's energy into them. If you're just getting started, a mound of firecrackers the size of an altoids box will deliver a decent whollop.
More advanced makers can use a pile as large as wireless router. But be advised, if the pile is too big you will have trouble generating the
compression required to ignite them all at once. And it can become unsafe at larger sizes.
Step 3: Location and Safety
Find a flat, hard surface to strike upon. If all you have is dirt, you might have better results by using a small tile. Earplugs are strongly recommended! Be safe!
Step 4: Detonation and Afterthoughts
With all of your friends watching, strike the hard surface with the end of the sledgehammer that has the fireworks taped to it.
Prepare for a boom!
Hopefully you have had fun and learned about the ideal gas law pV=nRT.
Also hopefully you still have your hearing and are safe and unharmed!