The BH float is used to do the final smoothing at the bottom of the trench.
Set the bottom of the bucket flat, then put it in float, it won't dig in, just scrape.
Mine doesn't have float but if I do crowd and lift at the same time I get the same effect and I can trim the curl as it comes in. Mine's an old 4-lever so I just pull the 2 left levers in together.