Basically it is a Indian Rupee prepay VISA card that you can use in shops, on-line or at any ATM in India,really a plastic alterative to travellers cheques but you already know what exchange rate you are getting
You can only put a maximum of 50,000 INR (about £575) on the card and you get the exchange rate for the day you load the card from your UK bank account, the card expires after 170 days and should you have any Rupees left on the card they are transferred back to your UK bank account, I assume at that days exchange rate