"Your entry is facilitated by your fighting stance.
Start by raising your elbows high like Thai boxers do. Then, when your attacker throws a punch or high kick, you can use your elbows to destroy his weapons.
If he opts to execute a low-line kick, perhaps one aimed at your groin or thigh, you can use your knee to intercept the blow and damage his legs.

The RAT System teaches you to envision yourself as a porcupine, hunkered down inside a protective layer of mobile elbows and knees. It doesn’t teach you to block, duck, or bob and weave. It implores you to destroy your opponent’s weapons right after he launches them.
The resulting pain provides the perfect moment to enter."

                                                   
                                                                                                     - Paul Vunak