In advanced practice there is no lean, nor is there any feeling of a lean, only a feeling of being centered and "sunk in." Due to the structure of the stance when one sinks it may only "appear" to be a lean when the spine is acutally perpendicular to the pelvis. Which is tilted slightly when sinking. There is a fine line between having a straight back and your power being "deadlocked." There is always a projection of the hara in a way, which is another way of saying there is always a potential for movement even in stillness. At advanced levels the lean detected is that potential. Very difficult to explain, let alone do properly. Very few can and do.