Gives Isaac a flat +0.75 tears even past the normal cap, -0.03 speed, and distorts the screen and background music. The effects can be stacked in the same room, and the distortion becomes more pronounced with repeated use. After repeated uses, the colors begin to turn very bright and intense.
Each use increases the recharge time, which starts at 0.5 seconds with the 1st use, and ends with 15 seconds on the 9th use. After the 10th use, it no longer recharges automatically though it can be recharged and used even more through batteries.
Leaving the room doubles the speed penalty and multiplies the tears bonus by x0.4, for the uses in that room.
Clearing a room removes one use's worth of penalties and bonuses, giving +0.06 speed and -0.3 tears.
Isaac's costume will change the stronger the Item's effects get.
This item belongs to the Fun Guy set. Collecting three items from this set will transform Isaac into a walking mushroom.
Rock Bottom: The distortion effect will wear off eventually, but the highest reached tears bonus won't. However, Wavy Cap will no longer have any positive effects until an even higher bonus is reached.
Book of Virtues: Creates a single mushroom wisp on the middle ring, firing 1.5 damage tears. Only one of this wisp can exist at a time.
Since the fire rate increase is flat, it is technically possible to reach the hard cap of 120.0 with enough uses. In practice this renders the game unplayable due to distortion.
On final stages of the distortion, the wave effect will make appear parts of the top of the screen on the lower screen and the lower screen on top of the screen, while bright colors dance on the screen, similar to how psychedelic trips are described.
If the player continues a save after maximizing the use of the Wavy Cap, they will respawn with the fire rate increase and changed appearance, but none of the screen effects. The screen effects progressively return as the player clears rooms again.