Over on the Collectors Society forums, a member cracked out encapsulated coins and posted videos showing how he did this. While I do not advocate cracking coins out of slabs nor do I condone it, the following videos show one person’s method.
Cracking a coin out of a PCGS holder:
Cracking a coin out of a NGC holder:
Regardless of the method you use, please do not hurt the coin. It would be terrible to see a coin destroyed during a crack-out!