See https://groups.google.com/forum/#!topic/comp.lang.tcl/5Jx7VY0ggC8
for original context.

The objective is to have support for event driven/background operations
on the ZIP file system in order to use e.g. fcopy with -command option
to asynchronously transfer data from a file in the ZIP file system to
a socket.

Initial implementation is in check-in [4d44284373]. It creates an
event source for TCL_READABLE/TCL_WRITABLE event support and borrows
most logic from the Windows tclWinChan.c module.

Optimized implementation using a hash table for tracking watched ZIP
channels is in check-in [880df261cc].