For sink washing, as for any textile washing, the main thing is agitation of the clothes. Knead them, squeeze them, put'em through the wringer so to say. Just letting them soak will not clean them well. You need to break up the fiber, make the fiber surface larger, to allow the water to wash out the dirt and the detergent to bond to the dirt. That is the function of the detergent. It is not there to get the dirt out (it does that, too) but to bind the dirt and prevent it from getting back onto your garments that are now in dirty water.

Till