The gemara is searching for a situation where one can be chayev misah bidei shamayim, and for stealing Teruma (since a non-kohen is not allowed to eat teruma). The case that the gemara comes up with is where reuven shoves Terumah into Shimon's mouth, and down his throat. But, if it was shoved into a place where Shimon can spit it up, Shimon is considered to be stealing as he begins to swallow it to a place where he can no longer spit it up, yet he is not chayev misah for eating Teruma until it is fully swallowed, so it is still not simultaneous. And if reuven shoved it into a place where Shimon cannot spit it up, Shimon should be exempt from both paying for the stealing and exempt from misah bidei shamayim since he was forced to eat it (Rashi and Tosafos). Therefore, the only case the gemara is able to come up with is where Reuven shoved it into a place where Shimon can spit it up "with difficulty". It is not very clear what this accomplishes regarding the gemara's goal of finding a case where the stealing and chiyuv misah are simultaneous. Rashi explains that the fact that he can only spit it up "with difficulty" is not really relevant, but since he can only spit it up with difficulty, it would have become disgusting as a result of reuven, therefore Shimon's requirement to pay is not for stealing the food, but rather just for the small amount of benefit he received by eating the food, which is simultaneous with the "eating" that would make him liable for misah bidei shamayim. Rashi writes that Shimon's liability is for "hana'as grono and meiav" - the benefit of his throat and stomach. Rashi seems to be referring to the gemara in Chulin 103b, where we have a machlokes whether for ma'achalos ha'asuros one is chayev for his "throat benefiting" or his "stomach benefiting". Therefore, according to the opinion that it is dependent on the throat, that benefit is what makes him liable both for stealing and eating, whereas according to the opinion that it is dependent on the stomach, that benefit makes him chayev for both stealing and eating. My question is that even the opinion of "hana'as meiav" seemingly agrees to the metzius that there is benefit in the "garon", just that it is not significant enough to make one liable for eating issur. However, regarding the stealing, he should be liable to pay for his "hana'as garon" at least a small amount of money, and that liability does not come at the same time as the liability for the misah bidei shamayim for eating Teruma (since hana'as meiav comes only after hana'as garon), so it is still not completely simultaneous?
The Ritzvah (in Tosafos) explains differently, that since Reuven placed it so deep into Shimon's throat that he can only spit it up with difficulty, the swallowing of shimon that makes him liable for stealing is so close to being at the same time with the eating that makes him chayev misah bidei shamayim, that they are considered simultaneous.