The trick to backing up on a steep hill is this.
put your left foot on the brake and engage reverse,
keep your left foot on brake and put your right foot on accelerator,
you have to accelerate harder than going on the flat and when the car starts to bite and go backwards, take your left foot off the brake.
The car might judder slightly if it is a particularly steep hill and I think this is the trouble you guys are having, but it is normal and is just another quirk of the smart

The first time I tried to reverse on a steep hill, I had a right job doing it, the car juddered and sounded awful, more accelerator helps, but it takes practice.
Ellis