Rush Mode is a recurring gameplay feature in the Star Ocean series. The term refers to the state that a playable character enters to perform an special attack when its Rush gauge is full. If other characters in the party have their gauges full, they can also follow the attack with their own Rush skills in quick succession, thus creating a Rush Combo (ラッシュコンボ, Rasshu Konbo?).
To perform a Rush Combo, one or more characters' Rush gauge must be filled first by continuously using the different skills a character is privy to. One must bear in mind that each skill also charges the gauge by different amounts, and sometimes certain moves are inadequate at generating gauge. In addition, the higher a combo count is, the faster the characters' charge their Rush gauges. This is usually not an issue in Multiplayer, but in Singleplayer, players that have trouble consistently keeping up a 150+ hit combo should make use of characters with Talents or various accessories to alleviate the issue.
In Rush Combo chaining, the order of the characters' attacks matters. Chains give increased damage bonuses, just like the Cancel Bonus for skill combos—the first Rush does base damage, the second does 150%, the third does 200%, and fourth does 300%. Generally speaking, Rushes with the highest multiplier should go last. As regards the characters' modifiers, they follow a general pattern: usually Healers have the lowest multiplier, followed by Shooters, Defenders, and then Attackers (with "finisher"-types being the most specialized in this department). Older characters also often have lower multipliers than newer characters until undergoing an Awakening.
When Rush multipliers are tied, generally speaking the character with the better stats and weapon (elemental or anti-race) should go later. Rush Mode bonuses (i.e., +X% damage for 20 seconds etc.) do not trigger until after the whole chain is finished, so these can be ignored for purposes of ordering the chain. In battle, the player can hold down on a character's Rush button to see what its multiplier and Rush Mode bonuses are.
Activating a Rush interrupts whatever the boss is doing and also wipes all projectiles from the screen. Several bosses have large area-of-effect attacks, easily telegraphed (but sometimes hard to dodge) attacks, or long-lasting projectiles that interfere with the players' ability to sustain offense. Depending on the difficulty, doing a solo Rush to cancel their attack can be a good idea (especially with a character who can easily re-charge their gauge, like Nel).
Rushes also cancel the Stun status ailment, so using a Rush on a stunned boss may mean the player effectively wasted a Stun. However, if the chain is going to kill the boss anyway, or if the party has a strong buff that is about to run out, it may not matter as much.
The total sum of damage that occurs during a Rush Mode is called "Rush Mode Damage".