Bhagavad Gita Verse 58, Chapter 2

yada samharate chaayam koormangaaneeva sarvashaha |
indriyaanindriyaarthabhyastasya prajna pratishtitaa || 58 ||
 
When, just like a tortoise withdraws its limbs from all sides, he withdraws his senses from objects, his intellect is steady.
 
yada : when
samharate : withdraws
cha : also
ayam : he
koormah : tortoise
angaani : limbs
iva : like
sarvashaha : all sides
indriyaani : senses
indriyaarthabhyaha : sense objects
tasya : his
prajna : intellect
pratishtitaa : steady
 
So far, Shri Krishna gave us a checklist of factors that could disturb our equanimity: joy, sorrow, gain and loss. Let's say, we detect that one of these factors has presented itself to us. What should we do? He gives a beautiful example from the world of nature to address this point.
 
Whenever a tortoise senses danger, he withdraws his limbs into his shell. The shell is strong enough to withstand any adverse situation. And once that situation passes, he brings his limbs back into the world. Similarly, if we detect that an object, person or situation is about to disturb our equanimity, Shri Krishna advises us to bring our intellect into the picture, and completely withdraw our attention from that object, person or situation.
 
In one commentary on this topic, we learn that most animals have one sense organ as their weakness. The deer has sound, the elephant has touch, the moth has sight (fire), the fish has taste, and the bee has smell as its weakness. So for example, if a moth sees fire, it loses all control and flies straight into the fire. However, human beings have all five senses as their weakness, making this technique all the more important.
 
The most practical application of this technique is dieting. If we have a sweet tooth like we saw earlier, and we see a large slice of black forest chocolate cake in front of us, we have to apply the "tortoise technique" and move some steps away from that cake. It also means that we do not keep large stocks of chocolates, cakes etc. in our house because we may be tempted very easily.
 
Here's another related point. In India, many aspects of spiritual practices are embedded in our customs, but sometimes we do not realize their significance. We may have noticed a sculpture of a tortoise outside many Indian temples, which is an instruction to withdraw our worldly matters and enter the temple with a devotional mindset. But instead of doing so, we tend to whisper about worldly matters into the tortoise's ear, which is exactly the opposite of the original intent.
 
The six shlokas beginning with this one comprise the answer to the third of Arjuna's four questions, "how does a person of steady wisdom sit", in other words, how does such a person control his organs?
Footnotes
1. The example on 5 animals and their weaknesses is from Vivekachoodamani by Adi Shankarachaarya