Proper TCAS actually sends out an interrogation pulse
to pick up the local transponder targets - cheap TCAS
relies on picking up the responses from secondary
radar interrogations in the vicinity.
I've got Skywatch in my beast and its amazing how
much traffic it makes you aware of that you simply
do not see regardless of the quality of your external
scan...
I was taught to go to standby as well, but my transponder
buffers an updated squark code until complete before
changing the old one.
-- Andrew