I'm looking for my first mower and I've found two different Gravely 36" walk behinds. One of the differences in the two is the reverse speeds. The larger one (15 hp kaw) says it'll do 3.7 mph in reverse. The smaller one (13 hp kaw) says it has something called reverse assist. What is reverse assist? Does that mean that I'll be "assisting" instead of the mower backing itself up?