The velcro piece (lenscoat) that attaches to the "zoom barrel" can be only 1/2 to1/3 fastened...
thus allowing for zoom action with some dust protection at the base, I have (2) 200-500's I've
shot in rain/heavy snow/dusty rodeos and have had no problems with either. HTH
FWIW, LensCoat changed up this design for zoom lenses with the one for the Sigma 60-600 S
It's a very light fabric sleeve that "gives" as you zoom in and out. Pretty lame in my opinion so
I had the Mrs. fashion me a velcro one with 2 adjustment strips and it works quite well.