The gemara has three versions regarding the type of shichvas zera that is able to make someone tamei. The first version is that only if his entire body feels the sensations, does it render him tamei. The second version is that any shichvas zerah that doesn't leave his body like an arrow, doesn't make him tamei. The third version is that shooting out like an arrow is only a necessary requirement to impregnate, but even if not, would render him tamei. The gemara explains that according to the first version, so long as the shichvas zera would begin to emanate with a hargasha, it would render him tamei even if it leaves his body without a hargasha. The gemara learns this from the fact that the shichvas zerah needs to be able to impregnate in order to be metamei. This implies that so long as the shichvas zerah begins with a hargasha, even though it leaves his body without a hargasha, it is capable of impregnating. Tosafos asks that this seems to contradict the gemara in Yevamos and Nedarim that say that if the shichvas zera doesn't leave his body forcefully, like an arrow, it isn't capable of impregnating. Tosafos offers 2 approaches: 1. The first version of our gemara holds that so long as it initially uproots itself with a hargasha it is capable of impregnating even though it leaves his body without a hargasha and is not "yoreh k'cheitz" - doesn't shoot like an arrow. In short, this gemara contradicts those gemaras. 2. The gemara in Nidah is only addressing the potency of the semen, but not the mechanical ability to impregnate. The fact that it uproots with a harghasha is the type of semen that is potent enough to impregnate, but the sperm will not reach it's destination so that it can actually impregante, unless it is yoreh k'cheitz - shoots out like an arrow. According to this second approach, the version in Nida that requires Yoreh K'cheitz would hold that otherwise the shichvas zerah doesn't have the chemical potency to impregnate.
According to the second answer of Tosafos, the third version of Shmuel that requires Yoreh K'cheitz to impregnate, would hold that it is a chemical determination of the semen, not merely a mechanical requirement of penetrating deeply into the woman's body. Based on this we can understand the gemara in Chagiga 15a. The gemara talks about the possibility of a man leaving behind sperm in a bath and a woman subsequently bathing and becoming pregnant. The gemara asks, how can this happen since the shichvas zera didn't penetrate her while being yoreh k'cheitz. The gemara answers that since it left the mans body while being yoreh k'cheitz, it can impregnate even though it didn't enter her body with the force of "an arrow". Clearly, the gemara understands this statement to be a determination of the type of semen that can impregnate, not a method of penetration into the woman.