as you add lye and solid powdery stuff starts to precipitate from the liquid, the alkaloids you are trying to extract are solidifying in the solution and droping to the bottom
after ading enough lye, no more solids will precipitate out of the solution
then all your alkaloids are in solid form meaning there are no more alkaloids in the liquid (robably a BIT more but not much)
thus you pour off the water as much as you can without disturbing the sediment, then add more water to "clean" up anything water soluble
repeat this until your alkaloids are a lighter c olor (usually 2-3 times)
then filter them and save the filtered solids
They don't think it be like it is, but it do.