Learn Finnish: Kun hän napsauttaa sormiaan, mielenhallinta loppuu. - When he snaps his fingers, the mind control ends.

You could grammatically say hän saa sormensa napsahtamaan, but it sounds like a literal, forced effort ('he manages to get his fingers to snap'). For direct physical actions you cause yourself, Finnish prefers built-in causative verbs like napsauttaa. The saada structure is generally better for causing other people or separate entities to act.

How does the verb napsauttaa function as a causative verb here?

Finnish frequently creates causative verbs by adding suffixes like -uttaa to a base verb. The intransitive base here is napsahtaa (to snap or click by itself). By using napsauttaa, you express the transitive action of actively causing that snap to happen.

Why is the object sormiaan in the partitive case?

Verbs involving the deliberate movement of body parts, like snapping fingers (napsauttaa sormiaan), waving hands (vilkuttaa käsiään), or nodding (nyökätä päätään), take a partitive object. The movement is viewed grammatically as an open-ended process rather than an action that entirely 'consumes' or completes the body part.

Why isn't the pronoun hänen used before sormiaan?