asterisk users - Jan 2005 - what does the "c" option in the zap phone number do

I was browsing the dial cmd page on the wiki, and followed up to the zap 
channels page. There are a couple of interesting (ok, so I'm a saddo) 
options, specifically the "w" (wait .5 seconds before dialling the
next
digit) and the "c" option (You may also use the special modifier *c*
to
allow for clear channel connections between PRI ports)

What does the "c" option do ?? Note, the "c" option is part
of the phone
number, not the "c" option that is part of the chanspec that means 
answer confirmation.

e.g.

dial(zap/cr1/12345) dial 12345, require "#" for answer confirmation
dial(zap/r1/c12345) dial 12345, clear channel connections.

I have tried to google, without success.

I would appreciate it if anyone could let me know what it "does"!

Julian.