If curious about the Rangers' mentality heading into tonight's Game 7 of their opening-round playoff series against the Senators, one need only to look back a single series of events, starting with about three minutes gone by in the third period of Game 6. In the din of Scotiabank Place in Ottawa, with the Rangers already leading the game 3-1 but their backs against the wall by being down 3-2 in the best-of-seven series, captain Ryan Callahan took a blistering slap shot from Chris Phillips to his left hand. He could no longer hold his stick, but stayed on the ice and nearly blocked two more. When he got to the bench, trainer Jim Ramsay examined him. At the next whistle, the Rangers were on the power play and out skated No. 24, no grimace on his face, just unwavering determination in his eyes. "I think it just epitomizes what this team is about," rookie Chris Kreider told The Post yesterday as Callahan skipped practice for what the team was calling strictly a maintenance day. "Guys are willing to do whatever it takes. I've been on a lot of teams growing up, and they all say they're willing to do what it takes, but they're not willing to back it up.