Doing something retrospectively means being conscious of what happened previously, or applying something new to past events. When you retrospectively comment on blog posts, you go back and comment on old articles.
If you speak retrospectively, you talk about past events, and if a museum retrospectively honors an artist, it often displays work stretching back through her entire career. If your raise at work applies retrospectively, that means you'll get additional pay from days you worked in the past, as well as the future. The Latin root of retrospectively is retrospicere, "look back."