Let's assume you have a dcd alias of code. You can switch to the subdirectory of code named opensource as follows:
dcd code/opensource
Which, for me will switch to /Users/mflower/code/opensource. Completion will only work for the "code" part of that url though. If I typed:
dcd cod
It would be completed to "dcd code". If I tried completion from:
dcd code/
Dcd would act like it had no idea what I was talking about.