When a check occurs away from the boards, it can be spectaculer.
Scott Stevens retired from the New Jersey Devils. He was the master at taking his opponents out!
As Karren said, the opponent has to have possession of the puck, or just got rid of it. If a check occures well after the puck was passed, the checker will be facing a penalty - interferance maybe.
If the checker throws a check, his skates have to be on the ice. If not, this could be a penalty like charging.
You know Darla, by this time next year you will be a pro. We'll be calling you hockey girl!