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I think there are 2 mechanisms that make what is called DWL work.
1) going slower.
2) The other one is really a side effect of DWL...
its a method to arrive at the top of a hill at a slow speed enabling
you to recover more of the potential energy on the downside. if you're going fast on the
top, then when you coast on the downside you wind up going too fast
and have to waste the PE by braking.
I think it useful to break the hill (or valley) up into the climb and the descent. I think the lowest fuel cost way up the hill is not too much different than the flats, fairly low speed in high gear. I see no advantage to DWL on the climb except to lower your speed. Arriving at the top at a relatively low speed would save gas. DWL is one way to get there.
For the descent it will obviously depend on the length & grade. Coasting being the best unless its too steep. The only place where DWL would make sense is if there is another climb coming up right away. If you need some speed to make it up the next hill without shifting down then DWL would be a technique to accomplish that.
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