Ha! If I could answer that I'd be a much better swimmer and probably a TI Master Coach. It was one of those magical days that happen every now and then. A bit more about it:
I had been attempting that 30 x 50m set, but was finding myself at 18 SPL to make the :43 pace for each repeat. Coach Stuart encouraged me to try lowing my SPL. I was skeptical (actually quite certain I wouldn't be able to make the times at 16) but gave it a try, with the results described above.
Now, reflecting on how swimming has been going for me lately with my 2-beat kick really transforming my stroke, I wonder:
It may have been that I was using a faster SR in my desperation to make the times on that set, and thereby curing an over-gliding problem I hadn't even been aware of. Less acceleration/deceleration in each stroke, more continuous speed. That certainly seems to be what is happening to me now.
But that set was pretty magical that day. I wasn't even tired, and as I recall, I would have kept on for another 20 repeats except the pool was closing and I needed to leave.