Can someone clarify the checking rule for me?
Are you allowed to bend your legs and drive upwards using your legs when innitiating the hit and said driving motion results in your skates leaving the ice (in the Backlund hit it was 4 or 5 inches)? That's exactly what happened in the Backlund hit.
Yes. Leaving the ice as a result of a hit is allowed. Leaving the ice prior to making a hit is severely illegal.
Players often go upwards after landing a hit because of how legs work. In order to make a hit, you need to thrust your body into it. In order to do that, you need to use your legs. In order to maintain balance, a good portion of that thrust from your legs is going to be upwards, not just forwards.
However, if a player is launching upwards and, for example, stretches their shoulder up specifically to hit the head on a hit where they easily could have gone through the body, that would be illegal. The problem in Backlund's case was that there wasn't really a reasonable way for Dumba to make the hit without hitting the head.