Withdrawals made from player account have no effect over the bonus balance. Meaning you can withdraw from player balance at any point and your bonus balance will be the same.
"Place bets equal to your bonus amount from your player account." So if I lost you will return my money like a money-back guarantee OR if I win you will just double my winnings? I think this is a bit complicated especially to less fluent English speaking people.
There are two separate balances: player balance and bonus balance. In order to be able to start using your bonus balance to place bets, you have to place bets from player account equal to the bonus amount, at a minimum odd of 1.8. Once the bets placed from player account that are required to activate the bonus balance for betting are settled (won or lost), you can start using your bonus balance to place bets, while you can also immediately withdraw any available amount in your player account, with no effect on your bonus balance.
When a player achieves the required rollover (6X) on the bonus balance, the entire bonus balance will be automatically transferred to player account, becoming available for withdrawal, etc.