A very common 1vs1 fight goes like this. Two planes from opposing sides see each other, and turn towards each other. As the two planes pass each other (guns blazing usually), one plane goes into a hard horizontal turn. The other plane does an immelman (pulls vertical until inverted, rolls out wings level). The plane that uses the vertical maneuvering, has he actually lost any energy? He's traded his kinetic energy into potential, right?