The saying goes: "it is easy to make bad wine from good grapes, yet you can't make good wine from bad grapes."
Severe pruning will:
1.- maximise the exposure of the leaves to photosyntesis
2.- allow air to circulate and prevent fungal diseases
3.- make the bunches/grapes more accessible for harvesting
And more importantly it will limit the output per vine to a few bunches full of flavour.
Fruiting shots are born in 1 year old dormant Buds. The aim is therefore to keep only a couple of 1 year old buds and get rid of the rest. Amongst them we look at the best placed for training along the trellis system used.
We avoid bigger/older canes which are called "bull" canes, thick in diameter, we leave strong looking canes at about the thickness of a pencil. We leave the two remaining spurs at 20-30cm in length.
Once you do it a few times it is simple: out with the old, in with the new. We were lucky to have a glorious few hours of sun during the wettest winter in Spain for years.