How is maildrop called from qmail ?

People,

I have had qmail happily operating for many years and in more recent 
years I have started using maildrop as the MDA:

   maildrop-2.8.3.20151220-1.fc24.x86_64

It just worked and I never really worried about it.  Now I am actually 
trying to understand how the two programs interact - maildrop doesn't 
run as a daemon and everything I have found just says that it takes its 
input from StdIn which doesn't help me understand the mechanism - does 
qmail attempt to deliver to the user's Maildir and somehow something 
alerts maildrop to take delivery of the mail?  I've grepped through all 
the /etc dir and the qmail files and there is no mention of maildrop 
anywhere . .

Any pointers in the right direction would be appreciated . .

Thanks,

Phil.
-- 
Philip Rhoades

PO Box 896
Cowra  NSW  2794
Australia
E-mail:  [email protected]