Air being sucked in will turn the compressor by itself without any help from the turbine, you just won't be compressing any air. I'd take the downpipe off and look at what is happening at the hot side while doing the same test.
i did this test with the turbo detached completely on all the cold piping, so the motor is running, and the turbo is just using exhaust to spin i would think. The intake and outlet on cold is operating without pull from the motor.
I'll pull the downpipe down, it's just attached with a V-band clamp, but what am I looking for on that side?