Someone smarter than me feel free to correct, but it seems to be a question of momentum. Taking off from a confined area, where your obstacle is at OGE height, you can get out with less than OGE power if you apply power early to gain an upward momentum and "coast" over the obstacle. The reverse is true, your downward momentum requires extra power to stop. If you start in an OGE hover of say 200ft, and descend at 500fpm with the intention of stopping at 100ft while still OGE, you will need greater than OGE power. How much extra power you need will depend on your decent rate and how much you weigh. I don't remember off the top of my head but I think the HAATS number for a Blackhawk is every 100fpm of decent rate requires 2% of excess power to arrest. If I'm right, it is easy to see how going from one OGE height to another with only OGE available could get hairy.
EDIT: What I'm talking about is a crappy situation that is not VRS but could make you think it was.