A clean cut block is where the blocker aims his shoulders at the defender's hips or even upper thigh, in a one on one blocking engagement. While this produces some bumps and bruises, it's still a valid and safe way of blocking. Those San Francisco 49'ers teams that won all of those Super Bowls in 80's and 90's used this kind of clean cut blocking under line coach Bobb McKittrick (one of the best in the business, I'd say).
The issue I have with Navy and Air Force, is that they weasel their way around to the maximum extent. They have no issues diving at someone's knees or ankles, and the asserting that it's a cut block because it had been a one on one block. They'll also chop block regularly, but the first blocker will release literally a tenth of a second before the second blocker hammers dives at the ankles or knees.
In the 1991 game against Air Force, an Air Force lineman dove right at Eric Jones' ankle, and almost severed his foot from his leg. The only thing holding his foot was his skin, since there was a catastrophic breaking of the ankle, many ligaments and tendons snapped, etc. During that same game, another Air Force lineman dove at Bryant Young's knee, which sprained it significantly to the point where he was out for a while, and maybe at 50% for the rest of the year. That really hurt us against Tennessee (yes, that big comeback game for them), since we were missing our two best defensive linemen.
2014 was an absolute disaster. Joe Schmidt was being engaged by a Navy lineman, while the Navy fullback dove at his knees. The lineman released from Schmidt less than a half second before the fullback made his impact of Schmidt's knee, and we lost him for the rest of the year. The Navy people were saying that because there was nobody technically engaging Schmidt when the fullback dove at his knee, that it was a cut block and not a chop block... There were a couple other guys who were also gimped after that game.