Tying prepares the grapevine for fructification and is usually performed at the same time as pruning.
In tying, the grapevine’s branches are bent and tied to living or dead props. This is the only way to keep the grapevine erect and allow the sap to reach all parts of the plant. Without tying, the buds at the end of the branches would get most of the sap, while the ones at the base wouldn’t be “fed”, thus making it impossible for them to develop correctly. 

How to perform?

The grapevine’s branches should be carefully bent or curved in order to prevent them from breaking. The branches should be bent on the third bud, but not too curved, since this will make vegetation too dense.