There are two types of Technical Machines corresponding two types of attacks in the game:
Fast TMs for Fast Attacks
and Charged TMs for Charged Attacks.
Types
Fast TM
Details TM that can be used to teach a Pokémon a new, random Fast Attack.
In-game description This Technical Machine item is used to teach your Pokémon a new Fast Attack. Your Pokémon will acquire a new Fast Attack and forget their old Fast Attack.
Charged TM
Details TM that can be used to teach a Pokémon a new, random Charged Attack.
In-game description This item works like the Fast TM but it teaches the Charged Attack instead of a Fast Attack. Remember, to use a charged attack you need to select the element button underneath the bar.
Trivia
Legacy moves are not able to be re-rolled.[1] (as in you can change a legacy move for another move but you cannot get that legacy move ever again).
TM cannot be used on Pokémon with a single move (e.g. Unown, etc.).[1]
Using a TM will teach a Pokémon a new random attack which is available for the Pokémon to learn and is not same as the attack that is being forgotten.
If the Pokémon has two Charged Attacks, trainer can select which move to be re-rolled.
Most Pokémon have two possible Fast Attacks, therefore it is guaranteed that those Pokémon will learn the desired Fast Attack.
Most Pokémon have three possible Charged Attacks, therefore there is ~50% chance of learning the desired Charged Attack.
