Anything referred to as an “internal martial art” tends to be a style that focused around using the intentions to lead movement, rather than just action and reaction. Internal martial arts also tend to draw their energy first from the Electromagnetic forces of the Body (or Qi) rather than using brute strength to apply their power. From a position of muscular system, internal martial arts tend to rely more on skeletal alignment and tendon strength rather than larger muscles movements for their applications. Internal martial arts tend to be more scientific in application and training methods, build fundamentals deeply before actually giving the meat of fighting applications to a student. This is truly a question that cannot be covered in a simple faq.
