HDSPMixer is the Linux equivalent of the Totalmix application from RME.
It is a tool to control the advanced routing features of the RME Hammerfall DSP
soundcard series.

Users of Totalmix shouldn't be disapointed because HDSPMixer presents the
same interface.
New users should read Totalmix documentation available from RME's website

As the ALT key is often used by window managers, the effect of ALT+Click and
ALT+Drag can be achieved using mouse button 2.