Based solely on large amounts of experience with Jr, I can tell you that HLD doesn't hit every time. Whether or not the opponent has DCI it has an effect.
If you have 46% HLD, then 46% of the time you'll lower the opponent's defense chance. Defense chance is determined by comparing your weapon skill level for the weapon you are holding (or magery shown skill if a mage weapon) plus any DCI items you have vs. your opponent's weapon skill for their weapon (or wrestling...or whatever) plus their HCI.
HLD lowers their defense chance by whatever predetermined amount (may be 25, I'm not sure) for a set timeframe (again, not sure), regardless of if the other guy is wearing any DCI or not.