This video of Amanda Knox was taken shortly after her arrival in her hometown of Seattle, Washington. In it she gives a short statement - thanking those who believed in her and supported her and her family. She was very sincere in her feelings of gratitude. One measure of sincerity in this context is Amanda's touching of her superior (upper) sternum and favoring her left upper chest. Our hearts are on the left side of our chests - and her repeated touching of the sternum and left chest is a reliable sign of this "Heart Felt" and sincere feelings of gratitude.
Please note, I have not seen any video and very few images from her trial - thus I haven't formed any Body Language/Nonverbal Communication opinions as to her innocence or possible guilt. Amanda's sincerity that I have written about above - is only with reference to her gratitude.