If you imagine metallic paint surface magnified. It is easier to understand the metallic pieces to be 3d triangular 'crystal' shaped - they aren't but you will get the idea quicker.
When you spray normally, they are there and they lay on theor sides, top down etc and this doesn't make the 'glitter' quite so intense, and some people even found metallic paints not to look that metallic at all.
You keep the last 'flat' coat tacky (feel free to flat down coats below this) When the penultimate coat is tacky, you raise the gun and 'dust' the last coat. Things brings the metal flakes - these 'triangular crystals' as we are going to call them into line. They float down and dry as they fall, they all land bottom down and stand upright.
Now you have super-glittery paint finish. You then lacquer over to protect and keep these in place with several coats of lacquer. The last coat of lacquer is then flatted down and polished up.
IF you were to flat the last colour coat, you would in effect, rub the tops off the triangular crystals - and destroy the metallic effect.
The actual shape of the metallic is different, but this is easiest way I can thing of you visualising it.