When Neville Stern first started solving, on Gary's Gems, he couldn't quite understand how a pinned piece could deliver checkmate. I was eventually able to explain it to his satisfaction. In reading Encyclopedia of Chess Problems: Themes and Terms by Milan Velimirovic and Kari Valtonen, I found out that there are a number of different types of composed chess problems in which that is the theme. "Loose Pin: A pin in which the pinned piece can move along the pin-line. This feature is thematically exploited in, for instance, Avner Mechanism, Bograd theme, Costachel theme, Cristoffanini theme, Leibovici Interference and Pelle theme." We'll look at a number of examples, over the next few weeks.
"Bograd Theme: The key piece unpins white thematic piece which threatens mate. Black re-pins it by a Pelle move, move by pinned piece, but also closes a black line, allowing the white thematic piece to mate also by a Pelle move, i.e. by moving along the pin-line."
Leib Bograd Wiener Schachzeitung 1937 #2
"Pelle Theme: All the moves in thematic variation(s) are made by pinned pieces."
Charles Pelle 10.cm Pat Ty 1938 #2
"Pelle Move: Move by pinned piece."
Isaak S. Birbrager Shakhmaty v SSSR 1950 #2
Giorgio Guidelli Good Companion 1915 #2
If you don't have a screw loose, you should be able to find the 'loose pins', and send me a complete variation to each problem, at firstname.lastname@example.org , by next Wednesday, for 8 points.