Usually you have a main attacker (whatever you want), draw power (claydol/uxie/etc), and an assortment of techs that aide you in your matches, whether it be tailored against a certain pokemon (such as ditto to help against gengar/flygon) or aimed to help your strategy (such as banette in Rayquaza C lvX).
Let's say you play flygon, and are having trouble against SP pokemon. You could TECH in mewtwo lvX so that they wouldn't be able to hurt you without pulling out a dialga G lvX. THAT'S what a tech is- something that isn't necessarily part of your attacking strategy, yet can help you in select matchups.
Another example of a tech is pachirisu GE (great encounters). It has an attack that discards tool cards, which is very useful against SP matchups, as it will get rid of energy gains and slow down the opponent. It also doubles as a decent starter, which is rare in the tech world.
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