The fulcrum is ideally in the smack middle of the grip (my preference), so a DL seat puts the reel farther from the fulcrum (middle of the grip) than an UL seat. Therefore, a lighter reel can be used on a DL seat.

Think of your fly rod perfectly balanced on your finger, in the middle of the grip. Now you move your reel from an UL position to a DL position. The reel moved farther from your finger so, unless you get a lighter reel, it will now tip back towards the reel. Does that make sense?

If you have a fixed balance point putting on a downlocking reel seat moves the reel back and increases the moment arm for the reel. That allows you to use a lighter reel to achieve the same effect as a heavier reel on an uplocking reel seat. You seem to be visualizing a moving balance point.

Any issues with a Proof order and one email it gets fixed. His cap and ring Seats are great and the Fenwick grips are on 3 of my old Fenwicks and most of my new builds from the past few years. Real nice stuff

Matt helped me out this past weekend. Broke off the end of the blank that I am working on by overtwisting, trying to get the reel seat off. I texted him (his # is on his website), and he called me shortly thereafter. On a weekend. We talked and decided an 11" stent will do what I need. He walked me through how to order the custom size on his website too. Can't say enough about the customer service experience. A+!