Your awning appears to be the A&E Dometic Weather Pro. If it is this is how it works.
Left Arm
Your A & E awning is designed to shed water if left out in
light rain. If water builds up on the awning, the left arm will
automatically decrease the awning height approximately 9
inches. When there is no longer the need to shed water it
will return to the normal height.
Have you tried it in the rain yet?
Electric awnings are great and anyone that thinks a manual is better is still in the stone age and seem to really hate changes.