Perhaps I'm not understanding the syntax. It appeared from the instructions that the splitting of features was not based on geographic boundaries as the old Arc/Info SPLIT command had been, but instead was based on a field in a table. In my case I have a statewide shapefile of census blocks, each with a county code, and I was trying to split the large shapefile into county by county pieces.
So "blk" was my "in_features" layer, but I also used it as my "split_features" layer as it contained the tabular "split_field" of the county code. And it did work to some degree, as I got separate output shapefiles for each county, but only a portion of the blocks rather than the whole county.
Is there another way to accomplish my task without having to select individual counties one by one and export the selected data? If I was working with Delaware's three counties I would have done that, but I'm in Texas where we have 254 counties.