This one is going to require... some lore.
Many of Sarcantasy's magic systems rely on the use of various Elements - which is not very original, I'll admit. You've got your Air, Earth, Fire and Water... but you also have other elements, like Wood, Metal, Shadow, and others that we'll get into along the way.
Dedicated study of any element can eventually make you an Expert at wielding it, which has some seriously nice perks. But only someone who has a natural affinity for an element AND works their butt off studying it can become Master of that element. Expert-level perks are seriously nice, but Master-level perks are amazing.
'Spell shadows' is a Master-level perk, which allows you to 'repeat' a spell you just cast. The repeat follows the same trail and has the same general shape, but is composed of pure shadow energy and deals only a percentage of the original spell's damage. As we can see here, that is still a very nice perk. A Master of Shadow can activate Spell shadows without having to say a word, and the Shadow element bypasses most barriers that aren't specifically raised against it.
The old man put up a pretty strong barrier, but not a universal one...