It returns a channel with cap - start queued integer events. (i.e. You can 'read' start,start+1..,cap from the channel and then it closes ).
If you poke around in the code, it uses a similar kind of construct to create an iterator for the the indices of non-zero entries of sparse matrices. Look in sparse.go.
It's not used anywhere in the code that I can find, it may have been just to test the idea.