Drain Rating and Drain Attacks
Each weapon in the game has a specific Drain Rating, which is the speed at which you will gain Ichor from hits on the enemy. The higher the value the faster you will gain Ichor. Generally speaking, weapons have a fixed Drain Rating per category (so most One-Handed weapons have the same value) but there are exceptions so keep an eye out for that.
There are five different kinds of drain attacks, and for those, the game will not use your regular attack value nor your weapon Drain Rating but the Drain Rating value of your Blood Veil (more on that below). On top of draining a lot of Ichor from your foe, you will also gain a maximum Ichor increase (until the Blood Code allowed cap).
Backstab Drain Attack: by backstabbing an enemy you will play a special animation depending on the type of your Blood Veil, during which you are invulnerable. Note that bosses cannot be backstabbed, but minibosses can. Grants +2 maximum.
Charged Drain Attack: holding down the drain attack button until it’s fully charged will let you unleash this attack. You are not invulnerable while charging it, and if canceled it will cancel the attack entirely. Takes a solid couple of second to charge. Grants +2 maximum.
Combo Drain Attack: a faster, lighter drain attack that can be chained after a regular hit. Grants +1 maximum.
Parry Drain Attack: Timing your Parry correctly will result in a similar animation to the Backstab, during which you are also invulnerable. Grants +2 maximum.
Aerial Drain Attack: Once in Focus you can use this attack, and should it connect it will play a different animation to the Backstab and Parry one, during which you are also invulnerable. Grants +2 maximum.
Ichor is an incredibly important part of the gameplay so I suggest familiarizing yourself with those attacks early on.
Some enemies have a special, long animation during which they are highly resistant to damage but not to drain attacks.