Triangulation - you know it when you see it!

Triangulation, as the title suggests, is one of those principles you won't often encounter in an actual game.  But, it's a cornerstone principle in understanding how to convert the most basic won endings where you're up just one pawn.  It's so basic that if you mess it up, you will be kicking yourself for ages.

Now this puzzle wasn't very highly rated, but there's a real nasty trick underlying it which made it really clever in my opinion.  I totally missed it!!!