It's called a Senlgrove Board, named after L.E. Snelgrove.  Covered
extensively in the archives, which can be searched at:
http://listserv.albany.edu:8080/cgi-bin/wa?S1=bee-l

Hits on Snelgrove are numerous (over 50).  Particularly recommended are:
http://listserv.albany.edu:8080/cgi-bin/wa?A2=ind9708D&L=bee-l&P=R1900
with a follow up a year later:
http://listserv.albany.edu:8080/cgi-bin/wa?A2=ind9812D&L=bee-l&P=R703

I am currently experimenting combining Snelgrove swarm control with Killion
methods for comb honey production.  So far (about 3 days into the
experiments) I'm happy with how things seem to be going, but it's definitely
too soon to tell.

Aaron Morris - thinking archives, search 'em early, search 'em often!