Product review (3 stars): Should be, but isn't, a mindless process.
As others have mentioned, this stopper can let some of the water drain out, if you don't set it in the disposal drain, just right. If you just put it in the hole, it may, or may not, seal. With a bit of care, making sure that the plug is seated very firmly, it does hold the water in the sink. Plugging a sink should be a mindless act, but, as said, you have to put in a little extra effort to get it done with this stopper.