It removes inactive users from downloads. It also removes all users from a complete download. .
Before:
After:
This can be very useful in aiding visual awareness of what exactly is going on with multi-source downloads.
This is a button primarily used if you have search results and items in the Transfer section of WinMX expanded.
For information on setting results to always be expanded Check out this page on how to set it as default. It also explains how to set many other of WinMX's standard Download settings.