A Tap Sequence is a combat occurrence that may present itself during any fight. It occurs occasionally when the player parries.
When Siris and the enemy lock blades, the word "TAP" appears in all caps at the center bottom portion of the screen, and the player must tap the screen rapidly. If, within a small time frame, the player taps the screen enough times, the player will break through the enemy's lock and the the titan will become stunned. If the player fails to tap the screen enough during the time period, the titan will land a free hit that cannot be avoided on the player.
This sequence can also occur against monstrosities, in which the enemy will attempt to devour either Siris or Isa. The player must tap the screen to hold back its jaws, and failing to end the tap sequence in time will result in Siris or Isa being eaten. Unlike other enemies, small increments of damage are inflicted against the player's character while they are in the sequence.
Each success against tap sequences increases the difficulty of the next tap sequence. After three or four successful tap sequences, the titan can easily outpower you afterwards even if you use multiple fingers to tap simultaneously. Monstrosities can also clamp onto you for longer before they can be expelled.