Lets say you go to a petrol bank and put fuel for Rs 100. You give the guy Rs 500 and you see him counting four 100 rupee notes to give back to you. He counts four 100s. He makes sure you see him count four 100s. He gives you the money. You think you've got Rs 400 back. In reality he has given you only three 100s. The other one by some sleight of hand is still with him. He relies on the trust he has established with you by counting so clearly in front of you to make you not double check the note count. If you trust him and put the money in your pocket without counting and leave - the guy pockets Rs 100. If you don't trust him and he sees you counting the money again and you reach the third hundred and give the confused look, he will be waiting hand outstretched with the fourth Rs 100. He pretends that he is giving you the last 100 separately. Like it was unintentional and matter-of-fact absent mindedness.
I noticed that this happened both in Bangalore and Madras quite consistently.
That leads us to an interesting story, which can only happen to the likes of me. There is a petrol bank in the intersection of Venkat Narayana road and Mt. Road. Its on Mt Round right before you take a left into V. Narayana road. My cousin and I were driving two separate scooters and we both put petrol for Rs 100 each. So total cost is Rs 200. I hand out Rs 500 and see the guy counting three 100 rupees. He hands over the money to me. I start counting it. There are only two 100s. I smile as I see him stretching out his hand a giving me another 100.
After we reach home I gloat about this to my cousin. Tell him how easily observable this ruse is. He looks a bit curious and asks me, "why did you pay Rs 200, I thought you only put petrol for Rs 100". And then I told him "thats because I paid for you as well". He then shouts at me angrily " you idiot! don't you have to tell me that. I paid Rs 100 as well"