You could set difficulty to zero, and you'd still be crawling up the hill. Probably slower! Take me, for example - I run 100% normally. When I'm free riding, I tend to do about 170-190w on the flats, but when I get to hills, I'll do 220-350, depending on how long the hill is. Then on the long descents it's more like 75-150w. Sometimes zero, yeah. If I were at 0%, I might put out a constant 190w. So, on a 6% gradient, the way I ride, at 250w, I'd be going about 8 mph up the hill. If I were at 0%, and just doing a constant 190w, I'd be going about 6.2 mph.
The Pace Partners provide an excellent example of this, at least as originally configured - running at a more-or-less constant 2.3-2.5 w/kg Coco Cadence is pretty easy to keep up with on the flats. Once you get to a hill, you naturally work harder, and you get ahead. Then on the descent you go softer, and if you're not careful she'll shoot past you.
__________________
"Don't take life so serious-it ain't nohow permanent."
"Everybody's gotta be somewhere." - Eccles