this can only be done at the end of the round
This depiction of the most iconic Pokmon definitely looks as if it was drawn before anyone knew that Pikachu would take off the way it did, because it looks little like the electric mouse that we all know and love
This means no attacks, fewer retreats, and in some cases, forcing your opponent to forfeit the match due to their deck having no way to overcome the cards that you place down
Otherwise, you can leave them in your trunk to be added later
Here is for example a live eBay listing for that same Japanese e-Reader, but the Pokmon cards are unsealed: Card e Reader for Nintendo Game Boy Advance GBA + Pokemon Card e SET Japan Used | eBay EDIT: Or is it just a sleeve the seller used, since the second picture seems to be from the same listing based on the background
Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities are triggered abilities and will have "when," "whenever," or "at the beginning of" in their reminder text