I hope this will help others understand coroutines.  A special thanks to miguel on the #tcl freenode channel for taking the time to explain this to me.  Many of the words used here are his.  If there any errors, i have injected them.

===
proc allNumbers {} {
    yield
    set i 0
    while 1 {
        yield $i
        incr i 2
    }
}

coroutine nextNumber allNumbers


for {set i 0} {$i < 10} {incr i} {
    puts "received [[nextNumber]]"
}

===

The first thing to understand is the [yield] command (which is in the coroutine's code).  [[yield $foo]] acts like [[return $foo]], but it suspends execution; when execution is resumed, the proc restarts on the line after the previous call to yield. ''MS notes that he kind of lied to jblz: [yield] is a bit more complicated, but behaves exactly as described for this example.''

The procedure "allNumbers" is a proc which first calls [yield], and so suspends immediately (line 2); when it resumes it begins execution at line 3, after which it reaches another [yield] within a while loop on line 5.  After producing each number, it yields its value ... and suspends until it is called again.

Now this line:
===

coroutine nextNumber allNumbers

===
This line will (a) run allNumbers until the first [yield] (b) create a command called 'nextNumber' that resumes execution of allNumbers when nextNumber is called.

So after this we have allNumbers suspended right before 'set i 0', and a new command 'nextNumber' that continues it where it left off.  

Get it?