Verb reduplication 伸伸 softens 伸 (shēn, to stretch/extend) into 'give it a quick stretch'. Very common in body-action phrases: 伸伸腰 (stretch the back), 伸伸腿 (stretch the legs), 伸伸胳膊 (stretch the arms). The second syllable usually goes neutral tone.
register
Casual, conversational. Often paired with 一下 or used in imperatives suggesting a small physical break.
