Just as Mountain Pose (Tadasana) is the foundation for standing postures, so is Staff Pose for sitting forward bends and twists. The "staff" is, of course, the spine, the central axis of the body, also called the "Staff of Meru" (meru-danda), a reference to the mythical Mount Meru at the epicenter of the Hindu cosmos.
In Staff Pose, you want the front spine to be long (but not necessarily "straight") and perpendicular to the floor. If the body is leaning back, it may be because tight hamstrings are dragging the sitting bones toward the knees and the back of the pelvis toward the floor. Most students need to sit with some lift under the pelvis, such as a blanket or a bolster.
A simple way to check alignment is to sit (on a support) against a wall. The sacrum and the shoulder blades should touch the wall, but not the back of the head. Put a small rolled-up towel between the wall and the lower back. Sit towards the front of the sitting bones, and adjust the pubis and tailbone equidistant from the floor. Without hardening the belly, firm the thighs, rotate them slightly toward each other, and draw the inner groins toward the sacrum. Watch the energy streaming up the front and down the back of the spine. Imagine the "staff" at the core of your own "cosmos," rooted firmly in the Earth, the brace and pivot of all you do.