According to Success Criterion 1.1.1 (Non-text Content), all non-text content, including images, should have a short text alternative, and (if required) a long text alternative. Technique H2 provides a method for remedying a situation where two links, one with an image and another with text, have the same text represntation. This results in screen readers "stuttering" over the link, when they should instead be presented as one link.
This is the typical example which technique H2 aims to address. HTML_CodeSniffer treats two links as an issue requiring H2 if the links are directly next to each other, or separated only by white space, and they have the same href attribute.