The SLR Magic Rangefinder is made to override the focus control of the taking lens with the Rangefinder’s built-in focus ring. With this function it allows single focus when shooting with anamorphic lens adapters that usually require you to not only focus the taking lens but also the adapter which renders the setup unusable in any real world situation. My ISCO has been sitting unused for the best part of 2years due to this reason.
However, the SLR Magic Rangefinder really changes this. Not only is focusing a lot easier, you get ZERO breathing during a focus rack!!! It's pretty incredible. The focus throw on the Rangefinder is 240 degrees which is cine lens territory. This makes focus rack really smooth (however, painful if you are pulling your own focus, even with a follow focus).
Downsides:
Can't use with a variable ND filter
Hard to use with a Matte box since the focus gear is on the mouth of the lens.
Anamorphic lens adapter: Isco Ultrastar 1.9x
Taking lens: Canon 100L
Camera: Canon 5D ML RAW