Just Attacks aka Timed Attacks aka Exact Attacks, is a special feature added into PSU and Ambition of Illuminus that allows you to increase your damage rate based on the time you press your attack button. Its not very hard to do, it just takes practice to get it consistantly.

To activate Just Attack, each weapon class has their own unique time for you to press the attack button. Sabers and Claws are the easiest to activate, but other weapons, such as, Double Sabers and Long Swords takes some time to get used to. You may want to start a mission and just practice pressing the attack buttons at different times to get the Just Attack effect. The key to getting a Just attack to work is to time your button presses. If you press any attack button before your supposed to,  your next Just Attack will fail and will result in a normal attack. Even if you mash the attack buttons but press it during the right frame of animation, it will not activate Just Attack. You can only press your attack button once for each combo at a specific time.

You'll know when Just Attack is working when you see a Yellow Ring around your character and that Extra Hard Hit sound from PSO plays.

You can string up to 5 combos of Just Attacks. You can also mix your Just Attacks with Photon Art: Just Attacks at any time during the 5 string combo. Note that the Photon Art (PA) will only allow up to 3 combos depending on the skill.  For example you can do a full combo in sequence:

Attack > Just Attack > Just Attack > PA: Just Attack > PA: Just Attack > PA: Just Attack. (6)

Or you can mix in your PA: Just Attack early in the combo sequence:

Attack > Just Attack > PA: Just Attack > PA: Just Attack > PA: Just Attack (5)

As you can see, once you input a PA: Just attack early on, your maximum combo is reduced. It will be reduced even further if you don't have your Photon Art skill level high enough to do a combo.

Finding the right time for each weapon will take some time. Here are some tips you can use to figure out the timing for the harder elements:

1. Double Sabers: The first Just Attack is initiated once your character finishes the animation and puts the weapon back to his or her body.
2. Daggers: The second Just Attack is initated when your character stops moving and is sort of crouching, but once they begins to stand up the combo breaks!
3. Twin Saber: After the second Just Attack, the first PA: Just Attack is initated after your character finishes the animation and is about to place their two hands together near the top of their head. This one is tricky because its a fairly long pause and if your a few milliseconds off the combo breaks.
4. Twin Daggers: After the second Just attack, the first PA: Just Attack is iniatiated right when your character is about to stand up. If you wait a few milliseconds too long and your character has completed the standing up animation, the combo is broken.