This is the reply I got from their support, just a few days ago:
>In short, our DDoS protection works by filtering out DoS-like traffic and is applied via the Linode network, so all Linodes are automatically protected. If your server were to be on the receiving end of a larger attack that impacts the Linode's host, we would need to prevent your server from receiving traffic until the attack ends. If you're concerned that you might be the target of a large DoS attack, there are a number of third-party DDoS mitigation services that you can use alongside your Linode.
>We aren't able to provide specific numbers since effects can vary depending on the attack. If you wanted to be sure your Linode is protected, we would recommend utilizing a third-party DDoS protection service overtop of your Linode's included protection. You also have the option of waiting to apply third-party protection until a null route is found to be necessary.
Edit: To clarify, filter = protection. Preventing all traffic is not. Both were stated in the description above so they should be clear which one it is.
Then I forgot to deposit a check at one point and overdrafted my account. I assumed things were fine because none of my transactions were getting declined. Instead I was being charged an extra $15 fee on every transaction, so that $0.75 stick of gum? $15.75, etc. This went on for about three weeks before I got my statement and talked to my bank.
They informed me that in fact the protection was from my transactions from being declined, at the paltry expense of $15 per transaction.