Rashi in Beitzah (16a) explains that the purpose of the neshama yiseirah, the additional soul that Jew gets on Shabbos, is so that a person should be able to eat more on Shabbos. I think that’s a very strange reason to get an additional soul.
Actually Rashi is saying something else. Rashi is saying that because of the neshama yiseirah we eat more on Shabbos – but not that it’s the purpose of the neshama yiseirah. The neshama yiseirah means that we have an additional type of soul on Shabbos that gives us extra energy and enthusiasm which is intended to help us understand more of the Shabbos lessons; so that we should feel it more. And therefore, because of that neshama yiseira, we’re more capable on Shabbos – if we utilize it, if we activate it, we’re more capable of learning these great Shabbos lessons and many other lessons too.
But because there’s an added energy now, so you’re able to eat more. That’s what Rashi is saying. When a man is in a good mood, you know he eats more; when he’s dejected, he eats less. And therefore when the neshama yiseirah enters and causes a man to be enthusiastic, so he has more desire to consume physical foods amd physical pleasures. But not that the neshama yiseirah is made expressly for the purpose of eating more.
What happens to the original soul when the neshama yiseirah comes?
The answer is it’s the same one only it’s better.
Now, you have to know that when we talk about the soul, we’re talking about an unknown quality. We’re like blind men talking about colors. And therefore we use only figures of speech. When we say a neshama yiseirah, we actually do not know what we’re talking about.
What we do know is that there is such an entity as a neshama – the Torah teaches us that very important principle at the beginning of the chumash. It says there that Hakadosh Baruch Hu breathed into mankind a spirit of life. He didn’t breathe it into animals. It’s only in human beings. And on Shabbos in a Jew that spirit is transformed in such a way that it’s called neshama yiseirah, and it’s capable of achieving that which the ordinary neshama cannot achieve.