This same "impossiblity" level, that, says, JAVA, VB or FOXPRO p-codes ...
. Every of them simply takes some worktime to understand bytecode structure.
Of course, high level languages are much better documented, as protectors.
One good method - create simple code, compile it to needed p-code (or, in our case, protect it) and examine result. After some similar cycles you will see, how this VM works and how commands are translated to p-code.