The user sends a transaction to a dedicated top-up address, and this balance is added to his account on LNDhub. Then, the user can use this balance to pay Lightning invoices. But under the hood, it’s actually LNDhub who pays the invoice, deducting user’s account balance. It works the same way when the user wants to receive a Lightning payment - it’s LNDhub who creates the Lightning invoice and actually receives coins on one of it’s channels.