It means your manpower won't plummet to +100 (from +300) in the midgame when you've got an average sized army. It may drop to +180 or so if you've got a huge army, but it will swing back and forth much less.
The "rubber band effect" of a losing player getting +300 and a winning player getting +100 will be greatly lessened. Players should be able to close out walk-over games easier and won't be stuck with low-tier armies... now they'll have the manpower they need to add late-tier units to their well-preserved early tier armies.
It's a wonderful and drastic change.
The math is so easy now that you can do it in your head.
Manpower income per minute = 300 - (population * 1.5)
Say your army pop is 60. That means your manpower income will be +210 because simply:
300 - (60 * 1.5) = 300 - (60+30) = +210
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