Current year wood (normally side shoots off the main permanent branch structure) is pruned back in the dead of winter to a couple of buds leaving perhaps 3-5cm of that years growth. The resulting shoots the following growing season are tied in and produce the flowers and grapes. If you cut back beyond that seasons growth into 2 or three year wood or beyond, you will just get growth and no flowers and fruit.

Incidentally, the reason for pruning in mid winter is there is no sap flow. If you prune in spring when the sap is rising, all the sugars the plant has stored during winter in its roots is pumped straight out of the cut ends and the plant can in severe cases bleed to death.

With some grapes, you need to leave a long shoot (6-8 buds) every 1/2 meter to produce the fruit. American Grape hybrids, grown for their hardiness and disease resistance, are notorious for this. All grapes will bear if you just let them run wild, but they become almost impossible to maintain, and the individual grapes become very small.