Of all the images in the Gita, this one may be the most universally recognized. A tortoise senses danger and pulls every limb inside its shell — head, legs, tail — all at once, completely. Krishna says the person of steady wisdom does the same with the senses. When distraction or temptation arises, the senses are drawn inward, away from their objects.
Even a child can picture this. The tortoise does not fight the threat. It does not argue with the predator. It simply withdraws. Quietly, instantly, totally. The Sthitaprajna's mastery over the senses has that same quality — effortless and complete, born of long practice.