This is the 12200th most frequent Spanish word.
involuntaria
Unintentional; done without intent or will.
The word 'involuntaria' is used here to describe an action (the smile) that was done without conscious control or deliberate intention.
Ella dio una sonrisa involuntaria al escucharlo.
She gave an involuntary smile upon hearing him.
In this sentence, 'involuntario' describes a reflexive action that occurs without volitional control, often caused by physiological reflex.
El movimiento de su pierna fue completamente involuntario.
The movement of his leg was completely involuntary.
Here, 'involuntaria' relates to a spontaneous and unplanned response prompted by an emotional or unexpected stimulus.