Hitlag

Freeze frames (also known as hitlag and hitstop) are a phenomenon that appears in the  games and Super Smash Flash 2. When an attack connects, both the attacker and target will freeze for a number of frames, hence the name "freeze frames". Characters will flinch while they are experiencing hitlag. Freeze frames are not present in Super Smash Flash, as characters will instantly enter hitstun upon getting hit.

Unlike in the official Smash games, the attacker and the victim can sometimes, depending on the attack, experience different freeze frame times in SSF2, giving one of them frame advantage. Pummeling also does not apply hitlag, like it does in the main Smash games.

The number of freeze frames applied depends on the attack. Weak attacks, like 's will have minimal freeze frames, but the freeze frames of strong, electric, and sweetspotted attacks can last as long as half a second. Freeze frames will also go into affect if two attacks clash or if the attack is perfect shielded.

Projectiles that apply freeze frames, on contact with an enemy, will not affect their owner. For example, ' movement will not be interrupted if her Charge Shot hits a foe. An attack can put the involved characters in hitlag even if said attack deals no damage or if the victim is invincible.

Smash DI is only possible while the character is experiencing hitlag after getting hit. It allows the player to change their position and possibly prevent opponents from comboing them.

In Super Smash Flash 2
The hitlag formula used in Super Smash Flash 2 is similar to the one used in Super Smash Bros. Melee, except it is about half the number of frames, plus one. The formula used to calculate hitlag is ${c ~ \times ~ \frac{e ~ \times ~ (\frac{d}{3} ~ + ~ 3)}{2}\ ~ + ~ 0.5}$, where $c$ is determined by crouch cancelling (2/3 if the target is crouching, 1 otherwise), $e$  is determined by the electric effect (1.5 if the attack is electric, 1 otherwise), and $d$  is the damage the attack dealt. For self-hitlag, the formula is ${\frac{\frac{d}{3} ~ + ~ 3}{2}}$.