could be a control emotions action. She was quite angry at the reporter and she wanted to look firm and in control. If both hands went flying around she would have looked out of control. She hid her left hand behind her back so she was hiding "matters of the heart" the left ring finger is a direct line to the heart so when people fidget with or hide it, they are signalling an emotional nerve was struck. Taking into account what she was saying, I believe she was trying to hide her false denial of hating Trump