You are correct in terms of what a clean cut block is, and that's when the blocker aims his shoulders at the defender's waist, hips, or even as low as the upper thigh.
You're mistaken on what a chop block is, though.
Chop blocks are when you have a blocker engaging the defender up high, while another blocker dives at the defender low while the first blocker is still engaged. This is a personal foul, and should be called every time.
Navy, Air Force, Georgia Tech (under Paul Johnson), etc., do a lot of what you would do with a chop block, but the first blocker engaged up high releases the defender just a split second before the second blocker's dive makes an impact. This way, the coaches can snidely say "nope, only one blocker hit the guy at the time of his injury, so it's ever so technically not a chop block." This is technically legal, but it's really, really dirty, and injures many defenders.
Often times, the first blocker engaged up high will grab onto the defender tightly, so that the defender can't move out of the way of the diving second blocker until it's already too late.
Link: https://forum.uhnd.com/forum/index.php?action=display&forumid=6&msgid=550962