Often this derives from a periphrastic form involving the active verb combined with a reflexive pronoun.
An example sentence is El padre se enojó al ver a su hijo romper la lámpara.
The (medio)passive is used when one is unable or does not want to express the actor: Čaša se razbila "The glass broke", implying it "just happened", almost "on its own".
The mediopassive is found in some contemporary Scandinavian languages like Danish, Swedish, and Norwegian (whereas for example Icelandic keeps up a formal distinction between the middle and the passive).
Classical Armenian had a mediopassive form that was marked by changing the verb's thematic vowel instead of with a unique conjugation like in other Indo-European languages.
Modern Armenian has retained some of these active/mediopassive pairs, but the distinction between the two voices is no longer productive.