The reflexive stem
The final stem—hithpael—does not have a active or passive counterpart because it itself lies somewhere between active and passive. That is, it is reflexive. In other words, the actor of the actions is acting upon himself. At least that is the idea in theory. In reality, hithpael verbs often have a reflexive meaning with respect to the base meaning of the word, but a simple active or passive meaning when the final inflected meaning is considered.
The best way to understand what this means is by examining the the most common hithpael verbs in the Hebrew Bible.
הִתְפַּלֵּל — “he intervened for himself (i.e. prayed)”
הִשְׁתַּחֲווּ — “they bowed [themselves] down”
הִתְהַלֶּךְ — “he walked about”
יִתְיַצֵּב — “he will stabilize himself (i.e. stand)”
הִתְקַדָּשׁוּ — “they consecrated themselves”
וַיִּתְחַזֵּק — “and he strengthened himself (i.e. took courage)”
וַיִּתְנַבְּאוּ — “and they prophesied”
With regard to the first example above, any sane translator will translate הִתְפַּלֵּל as “he prayed” and not “he intervened for himself.” Thus, you could say that this hithpael verb has a simple active meaning. However, we point out the “he intervened for himself” translation to demonstrate for you that, in fact, this hithpael form does have a reflexive meaning with respect to the base meaning of this word: “intervene.”
The same simple vs reflexive idea can be seen in “יִתְיַצֵּב” and “וַיִּתְחַזֵּק.” Here again, their are reflexive meanings to these hithpael verbs when their base meanings are considered. But the practical outcome are simple actions (“he will stand” and “and he took courage”).