Just sitting is the most easily misunderstood instruction in Buddhism, because it sounds like the absence of an instruction.
Shikantaza has no object. The practitioner does not follow the breath, does not count, does not repeat anything, does not scan the body, and does not observe thoughts in order to sort them into categories. Nor is it blankness, drifting, or waiting for something to happen. Teachers describe it as full alertness without a target: awake, upright, and not doing anything with whatever appears. The standard warning is against the two failures on either side of it, dullness and daydreaming on one hand, effortful concentration on the other.
Because there is no object there is also no measurable progress, and Soto tradition treats looking for signs of progress as itself the obstacle. This makes the practice hard to teach, hard to verify and hard to sustain without a teacher and a group, which is the practical reason Zen has always been institutionally heavy. It requires a container.
Rinzai work is structurally different. Concentration is directed onto a koan, the resulting doubt is deliberately intensified, and the private interview supplies a test. Rinzai teachers have historically criticised silent sitting as passive and liable to produce nothing whatever; Soto teachers have criticised koan work as goal-directed striving that reinforces the very self it is supposed to dissolve. That argument is old, is conducted inside the tradition by people who understand both sides, and is considerably more interesting than the harmony usually reported from outside. The Harada-Yasutani line of the twentieth century, from which a great deal of Western Zen descends, deliberately combined the two, which is one reason Western Zen does not map neatly onto the Japanese schools.