Three-Tails Chakra: Release

Due to his mysterious connection to the Three-Tails, Yūkimaru is able to at least partially control the Three-Tails. At first, Yūkimaru can only make an apparition of the beast appear, after taking some of Kabuto's pills. Later, after taking a larger dosage of the pills (combined with Guren's Crystal Release technique) he is able to make the beast fully appear, but couldn't control it. Later, when Guren is in danger of being harmed by the Three-Tailed Beast, he is able to halt it completely without Kabuto's pills. When Guren seemingly sacrificed her life to save him, Yūkimaru lost control of himself and used his powers to the fullest which enhanced the Three-Tails strength and rampage enabling it to break through the Four-Corner Sealing Barrier. This could mean that, like Naruto's jinchūriki forms, this ability is tied to his emotions.

Shizune concluded that due to the extensive toll this ability took on Yūkimaru's body, his lifespan was shortened, and he lost the ability altogether. 

Trivia

 * Yamato referred to this skill as "the ability to synchronise with a tailed beast".