EPTA great tvp example(but not really likely) is if a t is opening mass maruders and medivacs, so the z masses speed lings to counter. After you trade armies you mass helions while he still thinks your going maruders. And your helions>speedlings.
This is just a very simple hypothetical but the same metagame progresses through all aspects and stages of the game.
My final advice would be that it would be best to learn how to play solid and earn longterm wins with both strategies. Then during a longer macro game try switching, or even switching again and see the outcome. This could take 100 games to figure when it is in your best interest to tech switch. Most importantly in this stage of learning take the extra few minutes and watch your replay and see WHY you lost. Did you have poor macro? Did you not have enough rax/facts at the right time to properly export your resources? Did you upgrade the units BEFORE the tech switch? Did your opponent anticipate this tech switch and respond? Did you even lose because of poor unit composition, or was it because of other factors? These basics concepts stem from BW and almost everyother RTS. So try to learn the basics(what unit > enemy units) and proper mechanics and macro. The experiment on shaping a style of your play. GG
