NM Dan Heisman covers "defensive tactics" - sequences of moves need to prevent loss of material or checkmate. Some of the example problems are basic and easy - others are quite difficult.
NM Dan Heisman has been a full-time chess instructor since 1996 and is the author of 12 chess books, a TV show on Chess.com and radio at the Internet Chess Club. Howard Stern was one of Dan's students. Dan tries to answer comments on YouTube but for a quicker, more comprehensive answer (or questions about lessons), contact Dan via email, skype, or phone via Dan's website www.danheisman.com. His Chess Tip of the Day is @danheisman on Twitter.