Flinching is when you fly in the air and fall back to the ground in the Smash Bros. games. Most attacks have flinching. The only time you cannot flinch from certain attacks is if you use a flinch resistance sticker such as The Boss and Mother Brain (the two highest flinch resistance stickers, look to use those) in The Subspace Emissary. Primids also flinch from attacks. The only Primids that don't flinch are Metal Primids. Metal characters unlike Metal Primids flinch from attacks. You can always stop yourself after you are in the air by pressing a button such as the jump or attack button.