there are struts that lock the bed into place when you open the camera.
if you depress them again ( after the bed is horizontal ) they go to a 2nd position
and drop down maybe another 45º so the bed isn't in view of the lens.
in other words, instead of depressing the struts and having the bed go UP, you depress the struts
the same way, like you are folding it up, but you just guide the bed DOWN. not sure if that makes sense.
some say it isn't necessary with a 90, but i do it anyways.
good luck not needing a center filter! i have one for the sa i use, and it cost a kings ransom.
it eats 2 stops, but is worth every penny i paid for it ...