Well, here's a bit risky, but very good move to shaking Spits in Ki-84s.
When the Spit is about 1.5 behind you and closing steadily, with not too much E advantage to the Spit - lure it into a steep spiral climb to left side, keep going until speed hits under 175mph, extend the wonder-flaps and bank a little more, and start a insanely small radius flat-turn horizontally. By this time, the Spit is about 600 yards behind and cannot follow. While he is wallowing in the air, complete the turn and simply, you have his tail.
From the Spit's point of view, he first sees the Ki-84 about 1.5 in front, climbing into a very tempting spiral climb. It seems so easy to get the Ki-84 - as you follow him up the distance rapidly closes. However, as the speed gets lower, the Ki-84 will suddenly start looking as if its defying the laws of speed or gravity, and will start a crazy left turn with an incredibly small circle and radius. By this time the Spit's speed is nearing 150mph or under, and pulling any harder left/upwards to get a lead shot to the Ki-84 becomes impossible, as it will stall out the plane. The Ki-84 just plainly turns 360 degrees and lands right behind the Spit.
Works against all the Spits and the N1K. Doesn't work against Zeros and Hurricanes. Some excellent sharp-shooters can snipe the Ki-84 just as it starts extending flaps and shifts to the pure left-hand flat turn - but it's a pretty rare instance. Frankly, for an average pilot like me, this is about the most powerful reversal I can use against Spits.
It also works well against the Spit16. The Spit16 is powerful in climbs and verticals, but the Ki-84 still out turns it handily once the flaps are out, not to mention that the Spit16 is the worst turning Spitfire barring the Spit14. That's about the only decisive advantage the Ki-84 can muster against the Spit16. To use this move against Spit16s, you have to start the turn a bit earlier, so the Spit16 tries to grab a lead-angle gun solution and blows a lot of E.