All snarky answers aside, genre labels are often just a result of people's immediate reaction to it rather than some kind of predefined formula. What makes something "technical" metal? It's really your ears.
But if you need to put a fine point in it, I would say the amount of movement the composition has tends to be a factor, as well as the complexity of the arrangement. These days its hard for me to think of any technical metal bands that are outside the "extreme metal" fields, so if growled vocals and rapid fire drums aren't your thing, these examples are for you. As an example you could take a band like Nile or Obscura vs. the godfathers of grind Napalm Death.
In ND the guitar(s) and bass are almost always in unison, the guitar is playing pretty straight forward chords, and the bass is following the root. The movements are quick, but rarely break from that unison playing or run through scales as they pass through the root notes. Also, the arrangements of the songs are fairly straight forward, with some minor changes in tempo.
Nile does have a lot of unison playing, especially the bass which is generally buried in the mix. However, they have a lot of highly intricate runs through modes that are more exotic than just major and minor, the the composition of their songs is often so complex that if there is a "riff", they rarely use one more than once. Finally, and they incorporate instruments that are typical of the genre in passages of their songs that are borrowed from decidedly non-metal sources and blend them into their compositions.
Obscura is almost fusion jazz on a steady diet of anger and Red Bull. Their compositions and orchestrations are very complex, with both guitars and the bass frequently all playing different things at the same time. They play musical modes more common in jazz or classical in a metal way, with heavy distortion and blast beats, while not killing the feel of those modes.
Hope that helps.