You only washing it to get the majority of the trub out so you'll have cleaner yeast to make a starter.
Once you decide not to transfer to a new sanitized container you alow it time for the yeast to drop out leaving mostly cleared water.
At that time you can pour off the majority of the water...slowly so you don't disturb the yeast.
Depending on the container, you can leave about 1/4" of water on top.
At this point I'll swirl up the yeast from the bottom and pour the contents into sanitized baby food jars. Sometimes I get as many as 8 jars of yeast.
Make labels with the style of yeast -- if it has a number use it; i.e., WLP300, date, and which batch it came from. You'll need this in case that batch goes bad or is exceptionally good.