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maybe tl;dr but the qtractor wiki is currently the better source:
http://sourceforge.net/p/qtractor/wiki/Home/

have a special look on the 4.8 - Signal Flow chapter, please.

have special attention and focus on the diagram shown below, it tries to explain it all in one picture:)

1a. evil block diagram

1b. audio strip diagram

1c. MIDI strip diagram

a couple of threads around here might also give you some insights:

• Send bus effects in Qtractor
  

  
  2a. insert plug-in diagram

• Aux Send usage clarification
  

  
  2b. aux-send plug-in diagram

hth.
cheers