You're aiming to press button that comes after the colon in the notation (4A :A :A :A :A) just when previous attack's hit frames occur. This point in time occurs during one of the 60 frames per second that the game runs on. The game checks what inputs were given on each frame and if you input it at the right moment, you get a white flash and improved properties. This moment is the just frame or JF. Moves that require inputs on their JF are known as JF moves.Hmm... I guess I'm looking for the first white flash or w/e?
The colon usually shows where in the input the JF is and that's usually enough for anyone know they mean the JF version, but people don't want to type it all out all the time and if you don't space the letters out awkwardly you get 4AAA because of the : A : smiley.