Technical Machines, also known as TMs, are used by trainers to teach Pokemon certain moves they may not learn by leveling up.
The start of Generation VIII made TMs reusable, but Pokemon Brilliant Diamond and Shining Pearl will revert to the original functionality. TMs will be single-use once again, but all of them will be obtainable more than once.
Every Generation of Pokemon has had several TMs that teach Pokemon a unique attack. Until recently, with Sword and Shield, TMs could only be used a single time.
Pokemon Brilliant Diamond and Shining Pearl: How will TMs work?
Technical Machines in Pokemon Brilliant Diamond and Shining Pearl will have the same purpose as they have in every other Pokemon game. You can use them to teach a move to your Pokemon. It is as simple as that.
Interestingly, single-use versions of TMs are returning in the Generation IV remakes, rather than being used as often as possible or needed like they are in Sword and Shield.
Unlike the original Diamond and Pearl games, though, you can find multiple versions of the same TM. That much has been confirmed for Brilliant Diamond and Shining Pearl.
Obtaining a TM can be done in a variety of ways. They can be given by NPCs, received from Gym Leaders after defeating them, or found on the ground throughout the Sinnoh region.
With the Underground making its return as the amped-up Grand Underground, players may be able to find the repeat Technical Machines through mining under the earth.
That part has yet to be confirmed, so it is still a mystery how TMs will be acquired after you find it the first time. Finding them in the Grand Underground just makes the most sense.
Overall, it seems like the more things change, the more they stay the same when it comes to Brilliant Diamond and Shining Pearl. No matter what the upgrades are compared to the originals, there are just things like TMs being single-use that were bound to return.