.......Where there are Y-beams, I have used another approach that is similar. Then I have lines along the beams, with a butterfly loop at the correct position for the winches and no other attachment points than the winches. The eye is drawn around the winch one or two turns, under the rope and dropped over the winch.
A small boat will never exert any great power on its moorings, if jerks are prevented. The more common way of drawing lines directly from bow or stern to the jetty can generate jerks that cause wear and can tear cleats out of the deck.
Sorry Ink, but this method is not very good 
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All you have here are spring lines stopping the boat from moving backward and forward. What do you have to stop the boat from yawing? In this set up the bow or stern are free to yaw around and hit the pontoon.
I reccon you didn't try it then? There is no way the boat can jaw. It can move slightly sideways, a few inches, but it stays where it is. Opposing spring lines prevent jawing. The method is well tried as I said, well over thirty years. With the Y-beams, there is less yawing than when tied from a buoy, but also in the former case, jawing is no problem.
If it seems as the bow would be free to jaw, try to imagine a straight plank where the winches are attached instead of the boat, and see what kind of freedom it has to move. You can push that plank a bit toward any of the Y-beams, but you cannot get any end of the plank closer to or further from the jetty; you cannot twist it.
There simply is no power acting upon the boat to cause jaw. Any sideways power will act just about the same fore and aft, so a sideways drift will not jaw, but tends to move the whole boat sideways. When the lines are well stretched movement will be very small, only a few inches. Even if you deliberately force the prow to the side, the stern (and of course the rest of the boat) will move somewhat in the same direction.
People often overestimate the powers acting on a boat that is tied up, and at the same time don't bother a lot to analyse the geometry of the mooring lines. I agree, that for a large ship, this method might be infeasible and unnecessary, although for a small boat it is quite handy and evades one of the more common problem with moorings, that pitching will cause jerks in the lines attached at the jetty.
The only constant (or long lasting) forces acting upon the lines, not considering their initial tension, are those from wind and current. They are usually rather small. For my 23' boat of one and a half tonnes, they never become stronger than what can be countered by the puny power I can exert through my hands. Excessive powers however can be generated by swell combined with unsuitable geometry of the mooring lines. When tying up stern or prow to the jetty, pitch may cause jerks that exceed what the gear can take. I have seen fairleads ripped out, cleats too, and mooring lines broken, even when mooring springs were used to cope with the jerks.
There is also an alternative way to attach the spring lines, if you have an attachment point midships at the beams, when you can draw them to fore and aft cleats. Still just two attachment points, but now on the beams, with four points on the boat. It is perhaps easier to see then, that the starboard aft spring and the port fore spring will prevent jaw to starboard and vice versa. I have used this method when the beams were short and with a taller boat, so that the outer ends of the beams were about midships. Then springs from the prow to the end of the Y-beam as well as from stern holds it steady without jawing.
Just as with knots, testing will show whether it works or not.
