This mantle allows you to glide for when you don't have wings. 

As a Bonusaction you can open the wings and loose all fall velocity. Instead you glide up to 30ft. per round. While descending 20ft. for every 10 ft. of this movement that you spend. You gain no fall damage from touching the ground with the mantle open.  
If you try to turn the following rule applies: For every 90° you turn you spend 15ft. of the gliding movement.   
You can use a Bonusaction to close the mantle and loose every gliding speed and gain normal fall velocity. 

If you want to use an action doing something with your hands, while the mantle is gliding, you have to close it first. 
Spells that would push you, because of wind, that are directly targeted at the opened mantle push you three times the distance.

“The Only Distance That Matters Is The Distance Between Our Hearts.” - Rhondson, Tears Of The Kingdom