Interesting. There is a 'feature' in the 2004 Prius referred to as 'creep'. Austensibly so people wouldn't freak if they took their foot off the brake and the car didn't move forward (on a flat) as they would expect from any other automatic. If there is a balance of 'idle creep' and hill slope, it won't roll back. No different than any other automatic. The link shows a Prius and the 'hill holder' concept seems to generic to HSD. My '04 has slope sensors so maybe this is an improvement for the '06 (maybe '05, don't know) HSD. Another cause for '06 Prius vs '04 envy
Subaru has a "hill holding clutch". The brakes will stay on when you remove your foot until you start to release the clutch. Very helpful on hills. I needed to use the hand parking brake in my manual '86 Mazda for steep slopes.