After the goods are shipped, your supplier presents shipping documents such as invoices, insurance certificate and bills of lading to their bank, along with their payment instructions.
The supplier’s bank then forwards them to your bank for payment.
Your bank acts on behalf of the supplier's bank to collect payment from you - or a promise that you will pay at a later date if that's what you've agreed with your supplier.
A promise to pay usually involves you accepting a Bill of Exchange.
Please note, you'll need a specific bank facility with HSBC before you can request early delivery of goods.