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...

we now know that we need the word "atem" to teach us that partners, sharecropper guys, ect cant give terumah but if in one of the cases the person is a messenger is it allowed? like in a partnership one of the partners to give terumah is that terumah terumah?

Rashi on Terumah: Follow-Up

Akiva broached a blog discussion regarding using a שליח for giving Terumah. He discussed how a messenger gives 1/50th if he does not know how much the בעל הבית wants to give and that a generous man gives 1/40th, according to Rashi, and a not so generous person gives 1/60th, while 1/50th is for בניניו.
But he forgot to address the significance of the messenger in giving 1/50th. Question: What happens if a messenger accidentally (or even intentionally) gives 1/40th or 1/60th as opposed to the middle 1/50th? Well, the answer is that yes, it still counts. So we learn that a messenger is certainly effective in carrying out another person's מצוה to add on to the ideas from the פסוק of "גם אתם." This can be a very useful source for שליחות and whether or not it works for Terumah.

Akiva's post can be found:
http://feigenbaumtalmud.blogspot.com/2011/12/rash-on-terumah.html

Shelichos- What about Keddushin?

In the latest episode of Rabbi Feigenbaum's Awesome Shiur of Shiur Awesomeness, we are discussing how we know we can use a messenger, or Sheliach, for Keddushin. Originally we learned it straight from a Pasuk in Devarim, but the Gemara points out that the Pasuk in particular was discussing Gerushin, or divorces. We then learned that you can't learn Keddushin straight out from Gerushin, because Gerushin is Ba'al Korcha, against her will, and Keddushin needs her consent. From here on out, we applied Gerushin and tried to learn other Mitzvos with Shelichos, like Teruma and Korban Pesach, with Gerushin. However, I remember asking, "What about Keddushin? We learned it technically was different from Gerushin, so why can't we try to apply that to learn other Mitzvos can use Shelichos?" Truth is, we answered my question, I just flat out forgot it. So, what do you say champs? Do you have an answer to my question? Sound off on the comments below...

rashi on terumah

Rashi comments something very interesting regarding a messenger who you ask to do terumah for you. It says in the gemara that if you do not know how much the person who told you to give terumah (the ba’al habayit) wants you to give (if he forgot to tell you or something along those lines), then you gather according to what you think he would gather. There is one problem. What if you don’t know or have a clue what the ba’al habayit would give? The gemara answers that you should just give in the middle and give 1/50^th. I had an obvious question on this so of course I immediately looked in Rashi who apparently helped me out. My question was: How is 1/50^th in the middle, what is it in the middle of? Rashi answered me by saying that a good/generous man, then he would give 1/40^th, and a bad (or should I say not as generous) man, would give 1/60^th. Now, I understand what the gemara means by 1/50^th being in the middle: it is in the middle of what a generous and not generous man would give.

It is a good idea to all of my fellow classmates to look in Rashi when they have a problem with something because most of the time, he will answer your questions. In this case, he answered mine, however, we did go over it a little, after I looked at the Rashi, in class also.

question

I have a question. Why on earth do we need to know all of the details about schlichos. It is clear from the first piece of gemara that we can use one for kedushin, which is what the whole gemara is about. Once we already know that we can use a messenger for kedushin because it says so in the mishnah, we don’t need to deal with the concepts of schlichos in this masechet. My question is: If we are focusing on kedushin in this masechet, then why do we need to go through all of the halachot regarding schlichos, shouldn’t this be dealt with in a different masechet?

(This is a good opportunity for people to comment and answer)