Kneading does two things. First it mixes all the ingredients uniformly. You have to do this no matter what, but you only really have to do it enough to mix the ingredients.
If you keep kneading beyond the mixing stage, you are applying energy (which equals heat) to the yeast which makes it ferment, generating the tiny bubbles which make bread fluffy.
The yeast will ferment on its own, but kneading just accelerates that process.
Historically, dough was proved (left in a hot humid place) for about 18 hours allowing it to rise slowly in order to make bread.
In 1961 a process was developed in England called the Chorleywood Process. Essentially you work the heck out of the dough with high-speed mixers. The extra few minutes of high energy mixing applies heat to the yeast, which dramatically reduces the fermentation period required, allowing you to make bread much more quickly... at factory-type speeds. Factories can make bread in a couple of hours instead of having to prepare dough one day and bake it the next.
Poolish is a type of sponge. You prepare it before mixing your dough. As in 16 hours before. Then, you mix it with the rest of the ingredients.
What I usually do is mix 150g of flour and 150g of water with a pinch of yeast (less than 1g) and no salt at all the night before the day I'm going to use it. The idea is to use the same amount, in weight, of flour and water, and the least amount possible of yeast and let it work as slow as possible.
This will depend on many factors. I'm giving you my quantities, but it changes from winter to summer, you will have to experiment. That's half of the fun. For the mixing I use a fork and a 1 litter/quarter container and 30 seconds of work, it shouldn't take you more. The poolish usually gets almost to the top in 14-18 hours. Once I put 5g of yeast and got to the top in 1 hour. This is not bad but defies the purpose of the poolish taking a lot of time. Some people even put in the refrigerator to make it take longer.
Why would you want it to take time? This is what I was told and makes sense to me: when you mix water, flour and yeast two things happen: the yeast transform the sugars and some good bacteria starts creating acids that will determine flavour. The yeast work depends on the temperature and amount of yeast (among other things, like water, salt, fat, etc). The bacteria doesn't. Also, the bacteria and the acids help preserve the bread.
Once it has reached the top you have 1 hour approx to use it. If not, it will collapse and you better start with a new one. If you think you are not going to use it in 1 hour, use a fork to degass it before it collapses and let it rise again. It will rise faster this time, but that will buy you some time. You can see what it looks like.
If you have a recipe you like, try replacing 150g flour and 150g water with the poolish and see what happens. You may replace between 10% and 50% of the final dough. You'll see changes in color and flavor.
Best Answer
Well, technically there is a minor difference between biga and poolish, but often the terms are used interchangeably.
Just for clarification:
But should you explicitly knead?
Actually, you don't need to. Both pre-ferments will rest for hours. During that time the gluten will both develop (-> see the principles of no-knead bread) and be diminished to some extent by the growing yeast. You don't want or need to mess with this process. Just mix flour, water and yeast until somewhat homogenous (no dry patches) and let both yeast and enzyme activity do their thing. You will observe that the "uneven" preferment will change texture over time, yeast starts to bubble and when it's ready to use, it will be nice and even.
1 Yes, that's a really, really tiny amount.