Rack pulls are typically performed in a power cage. The bar starts higher than a conventional deadlift. This allows you to focus on lockout. You can also put the weight on blocks to make it start higher.
The "opposite" (in my mind) is you standing higher than the platform where the weight is. The bar is closer to your feet this way. This will improve getting the weight off the floor.
Hope this helps until someone who really knows what they're talking about can answer. I've done both of these, but I'm by no means an expert.