Tear height is a hidden attribute that determines how far above the ground tears travel, and in practice increases effective range. Very high tear height makes Isaac's tears' "cruising altitude" so high they can cross over obstacles as if they were spectral, but they won't go through the room walls. Once the tears fall down, they will start hitting obstacles again. This compounds with the starting height from the range stat.
Randomly while firing tears, Isaac will stop firing and turn red, where he will charge and release a lot of 'tears' in one go. The burst of tears also includes a kidney stone, which deals a lot of damage.
Boosts one of Isaac's stats depending on the color of the aura around him. The aura disappears upon taking damage and reappears with a random color upon entering a new room. Red boosts damage, blue boosts tears, yellow boosts speed, and orange boosts range.
Tears travel in a large spiral pattern and become spectral, with a small chance to home.
In co-op, Long Baby will occasionally briefly increase range, shot speed, and tear height. Dark Baby will occasionally briefly increase all stats, including tear height.
Azazel starts with -1 tear height, though the stat does not affect his short-range Brimstone.
Knockback is a mechanic that determines how far an entity is pushed back by a certain force.
There are two kinds of knockback: directional and circular.
This knockback comes from Tears, most notably Isaac's. The knockback force is determined by the speed and direction of the tear, which itself is determined by Isaac's movement and stats. For example, Ring Worm does not increase the knockback stat, but does affect knockback as the tear's speed hits extremes of very fast and very slow; hitting enemies with the fastest moment of its arc applies significant knockback. (Needs research and confirmation)
Directional knockback affects most enemies, including bosses (except for stationary ones like Pin or Round Worm). Items (both trinkets and pickups) cannot be moved by tears, except for live bombs.
Knockback from tears provides minor crowd control for homing enemies like flies or Gapers, increasing the time it takes them to reach Isaac.
Be wary of the natural knockback from tears when trying to dodge charging enemies (like the smaller twin of Gemini, Gurglings, and Leeches): The tears can push them off-track and towards the direction you are planning to escape to.
This knockback is a little shockwave that's emitted at a certain radius in all directions, most prominently from explosives, like Bombs. Certain farts also create a (non-harmful) knockback. Some enemies, like the Fatty, can fart, and Isaac can also do so with the Butter Bean.
Isaac himself is now immune to all damage, and a heart familiar follows him. If the heart is hurt, Isaac is hurt. The heart charges up as Isaac shoots, firing 8 blood shots, knocking back enemies, and leaving a puddle of creep when released.