Like others have said, it's a very early turn, initiated in the manner of a low-yoyo, plus impeccable timing. The greatest advocates that move can be seen in the PAC prime time, usually the Japanese pilots in their N1K2s.
Typically, the con you see up in front, is usually in reality much more faster than you expected. They come in with an alt advantage, but try to hide that fact. You usually see them coming in at co-alt, but in reality, they are coming straight towards you after a dive.
With the HO merge, they start an early turn with a slight dive, which if timed carefully, lands behind you about 400~500 yards.
To one's surprise, thinking that the N1K2 in front of you wasn't very fast, and just did a 180 turn, he sticks behind you at 400~500 yards and the separation isn't as high as you expected.
So, he lands behind you at about 450 yards, with about 100 yard separation per second - you are definately outrunning him, but not fast enough to outrun bullets. So, he sprays behind you and either you go down, or get scared by the dangerously close bullets and jink. If you jink, you lose the speed advantage and the separation becomes slower, or he even might catch you.
*Shrug* That's usually what happens. Is that realistic or not, I dunno, but you can do it against all planes with all planes if the timing and speed is right.