I'm not completely clear on what it means for a book to be "locked". Perhaps it's a Nook thing, but the Kindle has no equivalent.
When you get a new Kindle, you must register it and set up your account at Amazon.com - this is a simple, 10-minute task, which requires that you key in your Kindle's unique code number and you key in your credit card. This creates your
Amazon account. You don't have to enter your credit card again. It's stored in your account.
When I log onto Amazon.com from my home computer, it "remembers me". If I log onto another computer, I have to log into Amazon.com so that it can identify me. Once it identifies me, it "knows" my account information.
From that point, you can purchase books (either from the Kindle itself or through the computer). Each time you purchase a book, you click "Buy with 1-click", and one of two things happens:
A. If, when you make the purchase, your Kindle's wi-fi is turned on, the book appears on your screen in about 30 seconds. It is not "locked". You can begin reading that minute.
OR
B. If you buy through the computer while your Kindle's wi-fi is turned off (because it saves your battery life to keep it turned off), the book goes to your Archives. When you turn on your wi-fi, you hit "Menu", then "Sync for new items", and in less than a minute your book appears. Again, it is not "locked". You can begin reading that minute.
You get an email saying, "Thanks for your purchase of ____ book at the cost of ____ on ____."
Free book purchases are identical to paid book purchases -- except that no money changes hands. You see a book you want, the price is $0.00. You click "Buy with 1-click". The book appears on your Kindle. You get an email saying, "Thanks for buying ___ at a cost of $0".
I've had Amazon.com gift cards. They're easy too: You go to your account, enter in the gift card number, and you have a "credit" on your account. Say it's $25. When you buy a $5 book, your credit goes down to $20. Easy.
I did have one problem once: My credit card number was stolen, and the company issued me a new one. I had to "edit" my Amazon account, giving the new credit card number. Easy.