This is an ability a jinchūriki is able to perform when in a tailed form. The jinchūriki will use the raw chakra of the their Tailed Beast to form arms, which they can freely manipulate to attack their foes, able to strike an opponent faster than even a Sharingan user can react to. The chakra arms hands are clawed while the rest of the arm seems to lack any kind of joint, thus allowing them to have flexible movements that will generally be impossible for normal arms. The user can create any number of arms that they desire and they can also serve other purposes such as grappling hooks, to either grab onto objects or enemies from great distances, for offensive means, as merely touching one of these can cause fatal burns, due to the Nine-Tails' seething hatred within its chakra (though this is not always the case). Due to having chakra attached to his body, the chakra arms can grow in size and from the fourth tail up, Naruto, the arms, or any part of his body can divide into two or more fully operational appendages.Killer B has also been shown to use this technique though instead of arms, he can form horns which he uses for his Lariat technique.
In his Nine-Tails Chakra Mode, Naruto can use the chakra arms to a greater extent, as he can use and release chakra arms from anywhere on his body on a small or large scale. These arms even have the power to crush stone without Naruto putting much thought into it. In the anime Naruto was able to crush a sword with a chakra arm just by tightening his grip. After some practice, he mastered the chakra arms, able to manipulate various stones and geometric shapes without crushing them. Now he is able to use them in an attempt to create a new technique: the Tailed Beast Rasengan since he was advised by the Eight-Tails against the use of shadow clones in this form. Naruto's mastery of the Tailed Beast Chakra Arms is so great that he is able to create several at once to launch attacks at several enemies, use them to pull enemies into the attack, and even create tiny ones around his finger. The arms also grant him a greater level of dexterity and manoeuvrability out of what would normally be a helpless situation, such as guiding his Rasenshuriken during his fight against the Third Raikage. Not only that, Naruto can control each individual shroud he gives out to protect his allies.
Because he cannot use a tailed beast chakra shroud form like Naruto and Killer B, Gaara instead uses his sand control to shape existing sand into arms. He has shown similar techniques when he does slip into his Shukaku forms.