Gravity works as an equalizer; so with falling objects (going downhill), the tendency will be to negate the speed advantage that one of the two has. Also, when you're going downhill, you effectively have less resistance to push off against, but in turn, that means your useful effort actually goes down.
Going uphill is harder, but getting that better push off at the start will be rewarded more greatly for the whole of the subsequent flat-out run. It's the same thing in cycling; the uphills are what break up a group and can yield large (time-wise) breakaways.
And modern engines have less of a tendency to give out than a biker's legs, so once you start that pull out of, say, Juncao, it's relatively harder for anyone to pull up to you than on the flat, even with a slipstream.
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