The idea was to insert a suitably sized piece of bread crust with some crumb attached (unsliced loaf) into the press, crumb side up, and screw down firmly. This gave you two baits approximately a centimetre square, with a hole for the hook in each.
Once in the water, the squares would swell to sugar-cube size.
These things seem to be quite collectable now, but I had two in the '60s, both of which stopped working after the threaded screw jammed. I think they're best displayed on a shelf rather than used on the bank. They used to cost about 2/6 (12.5p), but sell now for up to £25 if the box is present and intact.