While applying for IPO, in the application form, you need to provide your VPA (Virtual Payment Address) in the Payment option.
On successful placement of your bid, you will receive a collect request on your UPI Mobile app for your authentication.
On successful authentication, the Bid amount will be marked as blocked in your account by your bank (as per ASBA guidelines) and your bid will be registered with the exchange.
On allotment of shares, the amount will be debited from your account else the amount will be released.
Note - Under the ASBA process, the amount is not debited from your bank account until successful allotment. Until such allotment, the amount will remain blocked in your bank account.