I think I get it now... I just had to imagine playing around with my sight level. Let me know if this is correct;

Assume first of all that the 2nd axis is set properly.

If the bubble level is perpendicular to the riser, and the riser is perfectly vertical, then aiming up hill or downhill will result in a bubble level that reads "level" if the riser remains perfectly vertical.

However, if the bubble level is NOT perpendicular to the riser, then aiming up hill or downhill will result in a bubble level that reads "not level" when the riser IS perfectly vertical. So in essence, your bubble level lies to you on uphill or downhill shots, resulting in left - right misses.

Since riser torque is introduced at full draw, you need to set the bubble level so that it's perpendicular to the riser when at full draw.

Is this right?