  Anti-fairy magic is the ability of anti-fairies to do some similar magic that fairies usually do like flying/floating and shapeshifting. But it's main use is to cause dark magic or bad luck.
Usage
Shapeshifting
Shapeshifting is one of the things that fairies and anti-fairies do in common. The only differences that fairies and anti-fairies do in shipshafting is their color. When fairies shapeshift, they leave their signature color. But when anti-fairies shapeshift, they turn into something that is black and blue and they dont have any signature color as shown in the episode "The Gland Plan " when Anti-Cosmo shapeshifted into a cow but he is not colored green instead he is colored dark blue and black.
Poofing
This is also one of the similarities of fairies and anti-fairies. They can both poof from one place to another or poof a thing from them. The only difference is when fairies poof it is called "Poof" but when anti-fairies poof it is called "Anti-poof".
Bad luck
This is the main use of the anti-fairy magic, to cause bad luck during friday the 13th. Every friday the 13th, anti-fairies(some) escape to fairy world and go down to earth to cause bad luck every where whoever commits superstitios beliefs. They often poof black cats infront a person's path or make people trip to spill salt. This bad luck ability of anti-fairies do not effect on people that wears or holds things for good luck. For example, in the episode "That old black magic ", Timmy's mom and dad wears things for good luck and they are not affected by the anti-fairies' black magic.
"Anti-fairy pregnancy"
Anti-fairy pregnancy is also caused by anti-fairy magic because anti-fairies' pregnancy is very quick. However, anti-fairy pregnancy is the exact opposite of fairy pregnany because instead of male giving birth to the baby, it's the females who give birth to the baby. And instead of vomiting purple vomit, they vomit butterfly vomit. And instead of feeling pains, they dont feel anything and actually enjoys it. See more information in this page Anti-Fairy Pregnancy.
Power
The anti-fairies power or magic source all comes from the Anti-big wand, which is the opposite of the big wand . No one is sure about how the Anti-Big wand generates power. Since the big wand generates power from human beliefs in fairies, maybe the Anti-big wand generates energy from humans not believing in fairies. The anti-big wand has onced loose power by Foop in the episode Anti-Poof, which he used the power from the Anti-big wand to make Anti-Fairy World into care beary world. See for more information " Anti-Fairy Wand ".
Weaknesses
There is only 3 known weaknesses of Anti-fairies that is seen in the show:
BUTTERFLY NET
In the episode Jimmy Timmy Power Hour: When Nerds collide 2, Jimmy used a hypercube with a butterfly net to catch the anti-fairies.This is one of the similarities of fairies and anti-fairies. Both of their magic are penetrated by a butterfly net.
GOOD LUCK CHARMS
Good Luck Charms, like a four leafed clover or a horseshoe facing right side up, is very helpful during Friday the 13th because this prevents bad luck caused by Anti-Fairy Magic . This weakness of anti-fairies is prooved in the episode "That Old Black Magic " when Timmy's mom and dad wears a good luck outfit and they are not affected by anti-fairy magic. See for more information: Good Luck Charms
TOO MUCH BAD LUCK
Sometimes, too much bad luck are also the weakness of anti-fairies. For example in the episode "That Old Black Magic" when Timmy does so many bad luck doings and the anti-fairies where tricked to went back to fairy world again. Also in the show When Nerds Collide, when Timmy and his other friends caused to much bad luck and the anti-fairies that should circle the globe have been tricked to come down to Earth because of too much bad luck and the anti-fairies have been sucked into the hypercube. This only happens when there is too much bad luck, and because of it, anti-fairies are easily tricked.