Leicester is a counter attack team but they actually counter with smart moves. They don't really smash and run.
Vardy's movement is insane. Drags defenders all around the pitch. And when he is trying to get himself open he's very difficult to cover.
It's a relentless attack on the backline. Vardy tries to get in behind, gets back onside, tries to get in behind again. He occupies so much attention and keeps defenders on the back heel. Guys like Mahrez come in behind him with so much space between the defensive and midfield lines.