For cash credit to card, you can select the option to pay with points on the OTP page of a transaction. Money will initially be deducted, but if you opt for the pay with points option, the amount will then be credited back to the card. Use the option for big amount transactions, as the redemption charges do apply for every transaction...
I usually prefer to redeem for AmazonPay vouchers. The best thing is that you can redeem all your points to the last 1 rupee. No multiples of hundred or anything for IDFC.