pruning Hydrangeas

We visited a nursery in Newbury, VT on Sunday: the Green Reaper--they had the most beautiful specimens of hydrangeas I've ever seen around here.  The secret is to prune them fairly heavily.  At my house, I have noticed that the ones which I use for cut flowers and therefore cut alot of blossoms, get many, many huge panicles.  Chris Esten, at the Green Reaper, said pruning is swhat gives hers such size.