Poison and Life Drain
Poison (via Bang's poison nail) and Life Drain (via Hazama's Overdrive) both steadily decrease the opponent's life while active, but can never kill the opponents by themselves; the most they can do is decrease the opponent's life to 1 health.
Hitting Two Opponents With the Same Attack
One attack can hit multiple targets assuming they are within range. For example one attack can hit BOTH Carl and Nirvana.
Normally, when an attack touches the opponent, the remaining active frames are converted into recovery frames. However, if there is a second hittable character, then the remaining active frames can still hit him!
This leads to visually strange situations like Ragna's 5B hitting Nirvana, then a moment later, Carl jumps into the active frames and gets hit. This is further compounded by the fact that hitstop only applies to the attacker and the target, thus allowing the attack to hit the unaffected character with the active frames plus the hitstop frames!
This even applies to projectiles like Jin's Hishouken, but since the projectile will shatter the instant it touches Nirvana or Carl, this effect is much harder to take advantage of.
When your character is invincible and he goes through an enemy attack, you may notice that a white circle will appear. This is to show that the character was invincible.
Note that it doesn't always show, for example, ground throws vs airborne opponents won't show this circle.