There are several features of this account that Hegel thinks raise his dialectical method above the arbitrariness of Plato’s dialectics to the level of a genuine science. First, because the determinations in the moment of understanding sublate themselves, Hegel’s dialectics does not require some new idea to show up arbitrarily. Instead, the movement to new determinations is driven by the nature of the earlier determinations and so “comes about on its own accord” (PhG-P §79). Indeed, for Hegel, the movement is driven by necessity (see, e.g., EL Remarks to §§12, 42, 81, 87, 88; PhG §79). The natures of the determinations themselves drive or force them to pass into their opposites. This sense of necessity—the idea that the method involves being forced from earlier moments to later ones—leads Hegel to regard his dialectics as a kind of logic.
I've read a little Hegel, and at its close it does seem at least preconceived, if not necessary. Is it similar to Wittgenstein's Tractatus, insofar as an idea is clarified into containing the others, so that we're just left wondering how we didn't know it already? Or is it because Hegel's dialectic ends in the absolute idea (if I remember correctly), rather than e.g. silence?