Similar to Limit Breaks in other Final Fantasy games, Overdrives are some of the most accurate and hard-hitting moves in the game. They help a ton in battle, and I like to charge up my overdrive gauges before boss fights. Unlike other Final Fantasy games where a character has to take a ton of damage or be near death to be able to perform his or her Limit Break, in Final Fantasy X, Overdrives can be charged a variety of different ways. Character customization is pretty sweet; what can I say?
Tidus' Overdrive is called Swordplay. Depending on the Overdrive, he does some hard-hitting sword moves to one or more enemies.
To perform an Overdrive with Tidus, you have to press the X-button when a moving cursor is in the center of your screen. This is harder than it sounds, because the cursor moves very fast, and you only have three seconds to complete it. The quicker you perform the Overdrive, the more powerful it will be. If you fail to press X at the right time before the three seconds are up, Tidus will perform his Overdrive, but it will be very weak.
You're able to perform an Overdrive each time your Overdrive Gauge fills up completely. The methods that are used to charge Overdrive Gauges are called Overdrive Modes. Here are the Overdrive Modes available to Tidus:
With so many choices, a player can get overwhelmed when deciding which Overdrive Mode to select for Tidus (or any character, really). The best thing you can do to make sure you're using your characters to their max potential is to take some time to figure out what you want each character to be in your party. Do you want Tidus to be a fighter? A healer? A mix of both?
How effective an Overdrive Mode is all depends on how Tidus is being used. Because of that, the best Overdrive Modes are different for almost everyone. (Ally is the one exception here, discussed further down.) Warrior is a great Mode for Tidus because he's fast and is a pretty decent attacker. However, it wouldn't make much sense to set Tidus on Warrior when you plan to just use him for support, casting Haste and using potions. The Overdrive Gauge would never charge, and the idea is to get Tidus to Overdrive as quickly and as often as possible.
Something else to keep in mind is that Overdrive Modes are meant to be changed often. As the game goes on, Tidus is going to get faster and stronger. Depending on whether you take him into Yuna's Sphere Grid or Auron's after filling up his own, you will be using him in a completely different way. His weapons and armor also change, making some Overdrive Modes more effective than others. If the weapon you have on Tidus has a chance to inflict a negative status ailment on an enemy, Tactician could be a great Overdrive Mode for him. Once you change his weapon out, however, Tactician could become ineffective.
I use Tidus as a support character when I play the game. He's never my main attacker, and he's never my main healer. His attack is nice though, and his turns come so often that I can spend a turn or two using potions or white magic on the party. Depending on what I feel he is doing more of, I alternate between Healer and Slayer early in the game. I start using Yuna more toward the middle of the game, and around that point I set Tidus on Victor and then Warrior (when it becomes available) because I do a lot of grinding, and I use Tidus mostly for attacking at that point since Yuna does most of my healing.
Ally is in my opinion the best Overdrive Mode for Tidus, and the one I stick with for the remainder of the game after I get it. I wish he learned it in fewer turns. Having it makes me feel good about the time I spent grinding with Tidus, though, because at that point Tidus' turns come around 2 to 3 times more often than other members of my party. With Ally, his Overdrive Gauge charges several times in a single battle, and soon it starts to feel like Tidus is in Overdrive all the time.