The Toto toilets are so good that even partially pressing the handle will give a flush. The problem is that the flapper when only partially opened gets sucked back down to the flush valve as soon as you release the handle. This only lets part of the water needed for a full flush go into the bowl.
To get a full flush you need to depress the handle all the way and release it. This ensures that the flapper floats free of the suction of the flush valve and fully opens for the proper amount of time to deliver a full dose of the water required for a full flush.