Shooting in one direction increases Isaac's rate of fire until the cap is reached. As soon as Isaac changes direction or stops shooting, rate of fire is reset to its normal amount.
Quote
"Intensifying tears"
Effects
Tear delay gradually decreases as the player shoots in one direction. Accuracy also decreases as the fire rate increases.
At max "charge", it will reduce tear delay by half, rounding down.
Rate of fire returns to its original value if Isaac stops firing, changes firing directions, uses an active item, or is hit.
Being hit, changing rooms, and using items no longer reset the Epiphora bonus.
During the delay when transitioning between rooms, picking up an item, or using an active item, it's possible to change the firing direction without resetting the bonus.
Synergies
Continuum - Reduces the need to change firing direction allowing Isaac to maintain the increased rate of fire.
Dr. Fetus - Firing bombs continually will decrease tear delay, although doing so is not recommended since it increases the risk of self-damage.
Ipecac - Firing bombs continually will decrease tear delay, although doing so is not recommended since it increases the risk of self-damage.
How to Jump - Changing firing direction while jumping does not reset the bonus, similar to when changing rooms.