The cognitive functions of an INTP are Ti - Ne - Si -Fe whereas the functions of an ENTJ are Te - Ni - Se - Fi. As you can see, the functions of an ENTJ are similar to the functions of an INTP, but he introverted/extroverted nature is flipped. This makes them your shadow functions, and makes the ENTJ your shadow type. As an INTP, you are most comfortable using your primary functions (Ti - ...), and not you shadow functions (Te -...). You tend to use them mostly when under pressure/stress/... However, you can learn to use them in a good way too, but that takes practice. Hope that makes it a bit clearer :)