The Atlas FR chainguide is designed to accommodate two chainrings and work in unison with a front mech.
The boomerang back plate is designed to fit between your bottom bracket cup and the frame. The guide can be secured into ISCG mounts, if you have them, to stop it rotating, but this isn’t essential.
Once in place, the lower roller arm can be altered to adjust chain tension, and the roller itself floats laterally to keep shifting smooth and maintain a better chain line.
There’s no interference with shifting whatsoever. There will always be noise and a little friction when using a roller rather than jockey wheel, but fully lubed up, it verges on stealthy.
With the bashguard in place the weight creeps up to 245g (without 145g), but the added protection and peace of mind this gives is well worth it if you favour slightly rougher trails.
The Atlas FR is compatible with 68mm, 73mm, 83mm and 100mm width bottom bracket shells.