Dim Mak is a Cantonese pronunciation of the characters for pressing/blocking and vein/artery. The 'Mak' can also refer to a Qi/Chi channel. These techniques are a sub-set of Qin Na that cause fainting, dizziness, blindness, paralysis, or death by striking nerve plexi, sealing arteries, or by blocking Chi flow.
I bring this information because it's important to note that this is not a "mystical art" and it is surely not Jean-Claude Van Damme destroying the "bottom brick" in Bloodsport. Dim Mak is a very real part of every martial art that shares the features or ancestry of Chinese fighting styles. If you are in a bar fight and someone grabs you in the soft tissue of your elbow you are now the proud victim of Dim Mak; You aren't going to die, this is no "touch of death", but you are going to be in pain, possibly debilitated from the pressing on joint, nerve, and artery. Hopefully the pain doesn't distract you because distraction leaves you open to all kinds of abuse.