You can pay down your mortgage either directly or indirectly.
With the direct repayment option, you make regular mortgage payments to BCV. These payments will be applied to the loan principal and interest, and your overall mortgage debt will decrease over time. However, your tax charge will increase every year since you can deduct only the interest payments.
With the indirect repayment option, instead of making mortgage payments directly to BCV, you put the money into a third-pillar retirement account that you pledge to BCV. This option offers some tax advantages: you can deduct the payments made to your retirement account from your taxable income; and you maximize the amount of your tax-deductible mortgage debt.