Tailed Beast Control

Throughout the series, several shinobi — jinchūriki or not — have demonstrated the ability to gain control of one or more of the tailed beasts. The cause and effect of this control varies between those capable of doing so. The only shinobi who have achieved complete control over tailed beasts are Naruto Uzumaki, Minato Namikaze, Killer B, Hashirama Senju, Yagura, Obito Uchiha, and Madara Uchiha.

Jinchūriki
The jinchūriki have the potential to control their tailed beasts' power, but require intense training and sheer willpower in order to properly wield it. According to Gerotora, whenever the jinchūriki draws out a certain amount of the tailed beasts' chakra, the beasts' very own psyche will fuse along with it, which usually involves an inner struggle between them as the jinchūriki goes deeper into their tailed beasts' forms. If a jinchūriki were to completely give into its influence, the seal will break and the tailed beast will be released. For the jinchūriki to have complete control, they must find something to fill in the void of loneliness within their hearts to give them strength.

While some jinchūriki, such as Gaara, Yugito Nii, and Utakata (in the anime), had shown to use their tailed beasts' power to a certain degree, to the point of even fully transforming into them, their control over it was lacking compared to Yagura, Killer B, Minato Namikaze, Naruto Uzumaki, Obito Uchiha and Madara Uchiha's skill, as they became the only known jinchūriki to have complete control.

Once the jinchūriki are on good terms with their tailed beasts, they can act as partners in battle. This advantage allows the jinchūriki to counteract genjutsu without requiring another person to be present, as the tailed beasts can disturb their jinchūriki's chakra flow to break the genjutsu. They can also transform into their tailed beasts without losing control and are able use to their considerable abilities to the fullest extent. However, to achieve the full transformation, the tailed beasts and their jinchūriki must come to an agreement about trading certain amounts of their own chakra, otherwise, if the jinchūriki has complete control over the chakra but does not have the tailed beasts' own cooperation, the transformation will result in failure, and the tailed beasts will then gradually consume their jinchūriki's own chakra whenever they use the tailed beasts' chakra, which can kill the jinchūriki.

In addition of having a trusting relationship with one another, the jinchūriki can permit the tailed beast to switch control of their body at will, allowing the tailed beast to act and speak through them.

In the case of the Ten-Tails, it varies from person to person. The beast itself will wrestle with its host in an attempt to take over their body by destroying their mind, as witnessed by Obito who had to overcome this near-crippling mental battle to attain complete control of the beast. Once control is attained, their appearance changes drastically: in Madara's case, gains a chakra-based mantle which has six magatama around the collar and a unique seal on the back. In Obito's case, the mantle was a part of his skin, which was covered in scales and gave him a more savage appearance. Along with this, nine orbs float behind them and they are able to manifest a khakkhara as well.

Non-Jinchūriki
Hashirama Senju could completely control the tailed beasts by forcibly suppressing their chakra with his Hokage-Style Sixty-Year-Old Technique — Kakuan Entering Society with Bliss-Bringing Hands, which utilises the power of his Wood Release kekkei genkai. He was even noted to have had several of the tailed beasts under his control at one point in time. Yamato would later also acquire this power due to his genetic alteration, however, by his own admission, he isn't able to use it to its fullest extent. Both Madara Uchiha and Obito Uchiha were able to use Hashirama's DNA that was integrated into them to exert control over the Ten-Tails during the Fourth Shinobi World War, by using roots generated from their bodies to connect themselves to the beast. However the connection, as stated by Madara, was not perfect.

The Sharingan has been noted to wield the power to control over the tailed beasts. This ability was first displayed by Madara and Obito, who can fully control Kurama. Sasuke Uchiha at first also used his Sharingan to suppress a small amount of Kurama's chakra within Naruto's subconscious.

When Obito learned of Danzō Shimura's use of both Wood Release and Sharingan, he speculated that Danzō intended to control Kurama.

The Rinnegan also has the ability to subjugate the tailed beasts by using its Outer Path ability. This was displayed by Obito when he gained the Rinnegan, and used the combination of chakra chains and chakra receivers to bind the tailed beasts into their reincarnated jinchūriki as well as manipulating their transformations.

In the anime, Yūkimaru was able to partially control Isobu, the former tailed beast of Yagura. Another anime-only character was able to help Naruto regain control of himself after the latter had been subjugated by the Nine-Tails.

Trivia

 * In Naruto 6: Road to Ninja, a jinchūriki can also have full control over their tailed beast if they share their tailed beast's feelings. This is shown when Menma shared the Black Nine-Tails' feelings of hatred and therefore became much more powerful with full control over it.