Why do you pour the ethanol SLOWLY in DNA extraction experiment?

I am talking about the standard DNA isolation experiment from kiwis and onions.Protocol goes something like this (general and conceptual version follows)

1) Chop the kiwi/onion up finely and add it to a solution of NaCl and detergent
2)Incubate at high temp for a bit
3)Now incubate at cold temp
4)mix well and filter
5)Add the extract to a boiling tube and add neutrase

Here's the bit I need explained

6)Slowly trickle x amount of ethanol VERY SLOWLY down the side of the test tube

Why do you add it slowly and why do you not add it directly to the solution?