‘Soft’ Isles can’t break Boychuk’s playoff cardinal rule again

Johnny Boychuk gets shoved by Alex Killorn.Here is the Islanders’ motto, as proven by the split of the first two games of their second-round series against the Lightning in Tampa last week, and spoken by defenseman Johnny Boychuk, one of two players on the roster to have previously won a Stanley Cup.

“If you sit back,” Boychuk told The Post on Monday, “you’re dead.”

So there is the directive sitting in front of his team with Game 3 on Tuesday night at Barclays Center. When the Isles were playing aggressive and physical in the early parts of Game 1, they were dominating the play en route to a 4-1 lead. They sat back in the third period, and quickly it became 4-3 before an empty-netter sealed…