The front of the tongue is pushed further forward and is held lower in the mouth when forming the 'short a' /æ/ sound than with any other vowel sound. The tip of the tongue will touch the inside of the bottom front teeth. The body of the tongue is rounded slightly upward. The jaw is lowered and the lips are held apart, allowing the entire oral cavity to remain open.
Source: https://pronuncian.com/pronounce-short-a