Cooperation ninjutsu (連携忍術, Renkei Ninjutsu), called collaboration techniques (コンビ術, Konbijutsu) in the anime, pertains to a branch of techniques where two or more existing techniques are combined to produce a much more powerful one. When initiated with the right timing, one will feed into each other to become a technique of greater power than the sum of its parts.
Although synchronised collaboration techniques has been seen used in the manga (such as the Fire Release: Toad Oil Flame Bullet technique), they were not explored much in the plot. In the anime, during the Three-Tails Arc, the concept of collaboration techniques is more explored upon. As Jiraiya explained to Naruto Uzumaki, greater power can be brought forth by two ninjutsu users who combine two different elemental ninjutsu. This can either be in destructive power, range of the width and/or range of the attacks length. If two techniques users cannot co-operate, then a collaboration technique is impossible.
Collaboration techniques are all about teamwork. Although collaboration techniques can be used by ninja who can simply match each other's timing, the best results are seen when collaboration techniques are implemented by users who are mutually like-minded and have a working understanding of each other's drives and character; the more time they spend together, then the more they can potentially become in sync with each other.
Ninja, such as Kiba Inuzuka and Akamaru, whose fighting style revolves around collaboration techniques, are close friends and thus have a natural disposition for collaboration techniques, as they can work together to execute a collaboration technique; as do like-minded characters, such as Naruto Uzumaki and Gamatatsu, who mutually are both 'simple' and 'stupid'. Once a duo of ninja have reached an understanding and established a bond, they will be capable of working with collaboration techniques.
The most elementary application of a collaboration technique simply involves two users delivering the same technique at the same time (i.e. Kiba Inuzuka's and Akamaru's taijutsu: Fang Passing Fang). This doubles the power of its preceding single-user technique, (i.e. Passing Fang). This would be a two-way synchronised co-ordination collaboration technique.
Some collaboration techniques work because one element is weaker than the other element. However, there are also elements that possess a good affinity with each other. Wind Release and Water Release also have been seen as having a good affinity for each other in a collaboration techniques. Naruto's and Yamato's Typhoon Water Vortex Technique is a wind and water combination that's created from the Wind Release: Rasengan and Water Release: Tearing Torrent. These would be examples of two-way synchronised elemental combination collaboration techniques.