Flinching (also hit-and-run or hit-and-hide) is a PvM tactic that entails a player luring a monster to a safe spot where the player can hide, attacking it once, and quickly returning to the safe spot before the monster can retaliate. The player then waits in the spot until the monster's health bar disappears from the screen before attacking it again, immediately retreating back to the spot, and repeating the process until the monster is dead.
If, however, the player attacks the monster before its health bar disappears or does not return to the safe spot fast enough, the player will risk sustaining damage. It is, therefore, important to only do a hit-and-run when the health bar has disappeared off the screen. This tactic only works with melee attacks, if you try to use range or mage, you will always get hit before you can hide.