| Renkei is also known as "Skillchain" or "Weapon Skillchain". This is done by having 2 or more Weapon Skills chain off each other to create additional damage. When you Renkei not only do you do more damage (rather than doing it alone) you also gain elemental damage bonus depending on the Renkei you and your party done.
The timing for Renkei is extremely simple, there must be an interval of 3 seconds between each Weaponskill.
To make a skillchain, one must start with a weapon skill in specific elemental groups, then follow with a weapon skill with a related elemental group. A skillchain chart helps show this better, one is avaialable in the Guides & FAQs section of the site. When combined, the two weapons skills will create a skillchain, and a mage can choose to Magic Burst on that skillchain, depending on the element of the chain.