RCS stands for Reaction Control Systems. RCS is a simple but effective tool. RCS lets pilots glide after accelerating. this can be useful if you want to go in one direction but shoot the other. For example, not many people know how to deal with Barracudas, the only ship in the game that uses the power called "dash". You will lose a fight with a Barracuda if you do not match its speed, especially if you are in an asteroid field where they are strongest. That's where RCS kicks in. You can just go the direction opposite of the direction you see them, turn around, and fire. If you have enough firepower, it will be almost impossible for your opponent to land a hit on you.
RCS can also be used against other ships. I always use it myself. It improves your agility, but most importantly, it increases accuracy and tips odds in you favor. More chances of dodging attacks. This aspect of the game is not just a tool. It can lower your vulnerability. This makes you harder to hit as most pilots find it a lot harder to hit moving targets than still. RCS is one of the least used tools in the game, which is weird, considering its strength and game advantage. While it serves a great advantage during battle, it can make mining in one place challenging. RCS is not always the best combination to go with really fast ships that don't have as much agility like the Shadow X-1 or Shadow X-2. This can result in crashing, out of control flying, and unintentional spinning. In times of battle and action, RCS is a good weapon. However, while mining, RCS is probably better turned off. But if you wanna use it, better be quick enough to turn it on when someone comes!