1) THE PENALTY FOR A YAVAM WHO DOES YIBUM PREMATURELY
QUESTION: Rebbi Eliezer maintains that when a Yavam did Yibum with a pregnant Yevamah who later gave birth to a baby which did not live, the Yavam must divorce her with a Get ("Yotzi b'Get"). Rebbi Eliezer maintains that the Chachamim penalized the Yavam for not waiting to find out whether or not the Yevamah was actually obligated to do Yibum.
RASHI explains that Rebbi Eliezer emphasizes that he must divorce her with a Get ("Yotzi b'Get") and he does not say merely "Yotzi" alone because he wants to teach that since, retroactively, the act of Yibum was valid (because the baby did not live and thus the Yevamah became obligated to do Yibum), a Get suffices and Chalitzah is not necessary.
Rebbi Eliezer's ruling supports the opinion of Rebbi Yochanan, who maintains that Yibum done with a pregnant Yevamah is valid. How, though, does Reish Lakish explain the opinion of Rebbi Eliezer? According to Reish Lakish, who maintains that an act of Yibum done with a pregnant Yevamah is not valid, why does a Get suffice, without Chalitzah? The woman still has an obligation to do Yibum or Chalitzah before she may marry others.
(a) The TOSFOS YESHANIM and RASHBA (and MAHARSHA) explain that when Rebbi Eliezer says "Yotzi b'Get," he means that the Yavam must give her a Get and perform Chalitzah. Rebbi Eliezer penalizes the Yavam by requiring that a Get be given in addition to Chalitzah. (Indeed, the Rashba's text of the Gemara reads, "Yotzi b'Get uv'Chalitzah.")
This is not the approach of Rashi, however, who understands that the Yavam divorces the Yevamah with a Get alone, and not with Chalitzah.
(b) The TOSFOS YESHANIM (in his second answer) and the TOSFOS HA'ROSH in the name of the MAHARAM answer based on the Gemara earlier. The Gemara (35b) explains that according to Reish Lakish, when the Mishnah states "Yekayem" (in a case in which the Yavam did Yibum, and then the Yevamah later gave birth to a stillborn) it means "Yachzor v'Yiv'ol v'Yekayem" -- the Yavam must first do another act of Yibum since the first act was not valid. Accordingly, when Rebbi Eliezer here says that the Yavam must divorce her with a Get, he means that he must first do another act of Yibum with her (since the first act was not valid), and then he must divorce her as a penalty for doing Yibum before he knew for certain that she was obligated to do Yibum. He may not do Chalitzah with her because he already had relations with her.
According to this explanation, why does Rebbi Eliezer say "Yotzi b'Get" and not simply "Yotzi"? The answer is that by saying "Yotzi b'Get," Rebbi Eliezer emphasizes this very point -- that although he must divorce her, he must first do an act of Yibum with her and acquire her completely, and then he must divorce her with a Get.