The purpose of the DISA is to raise the low and midrange torque "and" raise the top end power output. It is present on E36, E46, E39, E60 and Z4's with the M54 or M52 engines. It's a valve that opens or closes intake ports depending on the load.
Here is short video I made showing how to check the condition of the DISA and the various levels of failure.
Check back when I'll write up some tips and additional photos, on replacing the CCV, that haven't always been detailed on most other DIYs..