Tactically, Hit Lower Defense and Hit Chance Increase (HCI) appear quite similar. In fact, the two properties are entirely different. HLD affects the defense of what you hit, while HCI is your chance to hit in the first place. It may sound like the same thing, but it isn't. HLD requires you to hit someone / something before there is a chance for it to work. Whereas HCI works ever ytime you hit. The following example illustrates this using HCI, HLD, and DCI.
Example:
Opponent has max Weapon Skill (120) + max DCI (45%)
You have max Weapon Skill (120) + max HCI (45%)
Result:
You have a 50/50 chance to hit or miss.
But if you score an HLD hit, the next round is...
Opponent has max Weapon Skill (120) + 20% DCI
You have max Weapon Skill (120) + max HCI (45%)
Result:
For this example, you now have a much better than 50/50 chance to hit for the duration of the HLD effect.
Another way to look at it is that HLD stacks with HCI, once a target is affected.
HLD Proofing
An opponent with a cumulative Defense Chance Increase of 70% or more will nullify any HLD hit.