How to use the fan?

TLDR ; Shooting the side panel on the fan once breaks it open and lures nearby (usually 2) guards to it; shooting it again reverses the flow direction and blows air out of it, blowing anything in front of it away.

The mechanics of the fan work in two shots: one for luring the target(s) and shooting open the side panel, and one for severing the fan circuit so that it turns very fast in the reverse direction, blowing high speed air out.

Now it's very likely that the two guards nearby (one directly above the fan, outside the snooker room; and the one who scouts the corridor where the fan is) will be tagged curious and be lured towards the fan. If so happens, the guard above will lean along the glass railing, shooting which you get a domino kill. After that you may shoot at the panel again to change the fan direction and dispose off the bodies (for this it's best you shoot the panel again quickly after the kills before the corpses are packed into the jackets, because for some reason one of the packed corpses then will be stuck and not blown away).
Sometimes Spoiler:

(the man on the bench nearby drinking from a champagne bottle) will notice the scene and come over to inspect, but don't worry because that will be a bonus, you can guess why.

I found this to be the most scoring way to use the fan, and this is also not a time-bound kill, so you may proceed after completing other objectives, if any.