Walter Reeves just did a show on overseeding fescue and he says a slit seeder is just another name for a power dethatcher which may have a seed hopper attached. The one he used did not, but it had a big sticker on it that said Bluebird Rental. What he did was used the dethatcher in three passes, two in perpencicular passes and one diagonal, then spread his seed.

Ichabod

The difference is in the blades. A dethatcher uses flail blades where as a slit seeder (aka groomer or verticutter) uses delta blades. The flail blades tear up the turf leaving a huge mess. The delta blades make vertical slits in the ground.