I realised that the mute button on a send track actually disables the input to the send and not the output .
-Create a track , insert send device ( enable mute source ) and point the send device to send track 1
-On the send track insert a device that shows the input signal ( sadly renoise does not have that )
-Now mute the send track , the input activity meter shows no activity at all which means that the incoming signal is muted
Is there a reason for this imho odd decision ?