It was the HUFFNAGEL knot or at least a variation for tying a lure on with a loop.
After tying a simple overhand knot, go around once more.
As you tighten the knot it will turn into a figure-eight.
Feed the running end back through the figure-eight as shown then tighten slowly and adjust the size of the loop.
Tie a simple overhand around the standing line and tighten everything.