Overpayments Query

Karma

I'm currently in a good position where I can make overpayments on our HSBC mortgage. Looking at the mortgage information, it states the following:

You have an annual overpayment allowance of 10.00% during the Early Repayment Charge period. In the first year this will be based on the amount of the Loan drawndown and in each subsequent year on the balance of the Loan outstanding at the start of that year.

The overpayment allowance will be reset on the anniversary of Drawdown until the end of the Early Repayment Charge period. 

If you do not use your annual overpayment allowance in any year, any unused part will not be carried over to a subsequent year. You can make overpayments either by paying us more than the amount of your monthly payments on a monthly basis or you can make one-off payments from time to time. The Conditions contain more information about contacting us to set up regular overpayments and making one-off payments. An Early Repayment Charge may apply as set out in Section 8 - Early repayment above.

As we re-mortgaged a few months ago, can someone explain what 'Loan drawndown' is? I can't find any additional information on the 'Early Repayment Charge period' on this product.
Also, is this 10% of the entire mortgage? So if the mortgage is £100,000 and the repayments are £500 a month, I 'could' then pay a maximum of £10,000 (£833 a month) on top of my usual repayments.
Thank you