"Low end power" is simply how much power a kite with a given size are able to deliver when riding straight out, in low wind and relatively low board speed.
Some call it low end "grunt" also.
Got nothing to do with where it is in the window IMO, eventhough some kites has a different edge position true, but off topic.
Anyways, kites with low end power are designed with a curve and profile, so they deliver more power (pull in the lines) for the same kite size.
The reason why we dont want max low end power (so we can choose a smaller kite), is because it is on cost of windrange, as a good low end kite will always have very narrow windrange, so it is horses for courses.
Also, the heavier you are, the better and easier "low end power" kites can be handled and feels right, typically.
Of course you choose a bigger kite if you are heavier, but you will normally also choose a kite "type" with more low end, as you can handle it better.
If you are light, you will be tossed around in wind gusts if too much low end power, so here you want a kite with huge windrange instead.
If you got a kite with big windrange you will also be able to absorb gusts much better and easier, but you need a kite a bit bigger than if a "low end power" kite.
So whether the low end power of a specific kite is high or low, is important for you, for two reasons:
1. If you got an existing kite and has to add a new one, a different type - you need to know how the low end compare in order to match the size correctly.
2. In order to choose the right kite type for your liking (f.ex waveriders typically dont want low end power but windrange) and your weight as told above.
Hope this explains ?