First, they see how the oobleck reacts to a chainsaw, will it respond like a solid or a liquid? When you take a chainsaw to a balloon filled with water, the water simply falls straight to the ground, unaffected by the rotating chain. The non-newtonian fluid however will react like a solid under the force of the chain shooting forward and then fall to the ground like a liquid.
By far the best part of the video comes when they decide to shoot a golf ball at over 300 feet per second at the balloon, and it is quite a sight to see. Upon impact, the fluid reacts like a solid and sprays in every direction, quickly returning to a liquid state and creating a big mess in their backyard. If you want to make yourself some non-newtonian fluid and play with it at home, all it takes is some water and cornstarch!
Physics is awesome, and sometimes even though you know the math behind how something will react, you never expect it to look as awesome as it does. Seeing a liquid turn into a solid during an explosion, then turning back into a liquid as it falls to the ground is pretty awesome no matter who you are.