What is an authorisation hold?
The petrol pump took 5,000 rupees for a 2,000 rupee fill, and both numbers are correct.
The money has not moved. A fence went up around it, and nobody is in a hurry to take the fence down.
Why anyone would do this
Some businesses cannot know the amount at the moment they need the guarantee.
A petrol station does not know what you will pump until you have pumped it. A hotel does not know whether you will empty the minibar. A car hire firm does not know whether the car is coming back with a scratch. Each of them needs an assurance before handing over the goods, so they ask your bank to reserve a plausible maximum.
Your bank agrees, and reduces your available balance by that amount. Note the word: available. Your actual balance has not changed. A number that governs what you can spend has.
Why it takes days to come back
This is the part that produces genuine anger, and the explanation is the three clocks.
Authorisation is fast. Release is not. When the merchant finally captures the true amount, the excess is supposed to be released, but the release travels back down the same slow rails as settlement. Between the hold and the release sit two banks and a batch cycle.
If the merchant never captures at all, the hold expires by itself, typically in seven to thirty days depending on the card and the category. Nobody is holding your money maliciously. They are simply not in a hurry, because the cost of the delay lands entirely on you.
What actually helps
Debit cards hurt more than credit cards here, because a hold on a debit card eats real spending power in a real account. If you are hiring a car or checking into a hotel, a credit card absorbs the blow.
If a hold is genuinely stuck, the merchant can usually release it faster than the bank can, because the merchant is the one who has to send the instruction. Asking the hotel is more effective than asking your bank, which is the opposite of what everyone tries first.
A cloakroom that takes your whole wallet as a deposit for one umbrella, promises to give back everything except the umbrella fee, and then goes on lunch.
Where you meet it
Petrol stations, hotel check-in, car hire, and every free trial that took one rupee to prove your card exists.