This is more of a momentum issue. Take a rope and tie it to the front of locomotive. Now let the train move forward at 60 mph. When the train arrives at point A, you give the rope a yank to help pull the train. Did you make it go any faster? Not really.
My example in my previous post works the same way here. Rather than pushing the marble up the rubber mat, we push it down. Put the marble at the edge of the slope and push it down. The marble accelerates to the bottom. Back the marble up on top of the slope and push it with an initial velocity. The marble goes down and speeds up and finishes faster than it was when you just pushed it down the first time. There will be a point that when you give the marble an initial velocity fast enough, that the slope of spacetime will give a neglible increase in velocity.
Take riding a bike for instance. We'll have three slopes, a 1, 15, and 35 degree slope. You start at point A which is 1000 feet from where the slope begins to go downhill, point B. Each hill is 1000 feet long and ends at point C.
Lets say you are a pretty good cyclist. You start at point A and accelerate to 20 mph to point B. Your speed will be so great that the acceleration you should be getting from gravity from the 1 degree slope is neglible. So when you arrive at point C, you are still going 20 mph.
Take the 15 degree slope. At point B you are moving at 20 mph, and because of the greater slope, you would accelerate to say about 25 mph at Point C. However, if you got to point B going 30 mph, you wouldn't have had an increase in speed at Point C because gravitation assistance would have been neglible.
Take the 35 degree slope. At point B you are moving at 20 mph, and because of this slope, you would accelerate to say about 30 mph at Point C. However, if you got to point B going 45 mph, you wouldn't have had an increase in speed at Point C because the gravitation assistance would have been neglible.
This speed where gravitation will not assist in speeding your bike up depends solely on the slope of the hill. The steeper the slope, the more speed you can accelerate. The slighter the slope, the less you can accelerate. We only accelerate towards earth because of spacetime being warped. This is what controls our maximum velocity given through aid of gravitation.
That should take care of that.