The frame is a cover meant to protect the head from an incoming strike. It’s best against strikes coming from the side, such as a roundhouse punch but it often deflects straight shots as well. It’s possible to both single frame or double frame, which simply means cover with one arm or two.
A good frame keeps a window in the house same way a good frame keeps the brain in head. The arm should be tight against the head. If there is space between the arm and the head then there is room for a fist or foot to slip in.
Because a frame is not a block but an absorbing cover, in an ideal situation one would actually move towards the strike so that there is less momentum when it hits. As anyone who’s blocked a hard hit will tell you, it still hurts.
Some key points mentioned in the video: keep your other hand up (in the single frame) and keep your feet underneath you so that you can deliver an effective (quick) follow up if needed.