Shelichos- What exactly is BiMachshava?

Further discussing Shelichos, we learned that we can't derive Shelichos in Gerushin/Kiddushin from Teruma because Teruma is BiMachshava. BiMachshava means you can do it with a thought process, meaning that it requires less action that Gerushin/Kiddushin, so we weren't able to derive those two from Teruma. But what exactly is BiMachshava? It doesn't necessarily make any sense? Thankfully, we have Rashi around. Rashi explains to us that by BiMachshava, one can look at a portion of his crop and think, "This is my Teruma and now I can't eat it." Note: This is later seen in Korban Pesach, where somebody can look at a lamb and think "That is my Korban Pesach." So, my question is, what exactly does it mean that you can just "think" and do the Mitzva. Since when can a simple thought determine how a Mitzva is done? How much thought is required, and how can you measure it? Is there a certain amount of Kavana needed? And what other Mitzvos, other than Teruma and Korban Pesach, can be done through BiMachshava? These are good discussion questions, and hopefully y'all will be able to answer in the comments below...