In your inventory, go hover over the elf’s Spear or a Shield + Weapon combo, you’ll see a blue arc that covers a full 180° semi-circle. Hovering over other weapons will show correspondingly smaller arcs. Some are quite terrible, see Dual Daggers, for example.
Enemy swings that are blocked from inside that arc are blocked at normal cost, and ones that come in from outside that arc cost double the stamina - and some monster attacks seem to deliver damage through out-of-arc blocks.
Pushing is weaker against enemies that are outside your arc as well, and they’ll be staggered for a lesser period of time.
If you equip two effects that give +30% effective block and push angle, then that example 180° arc is now a 288° one, leaving only a 72° arc directly behind you that is weak.
The difference can be most easily felt by loading up Handmaiden with +stamina regen aura along with the talent that improves stamina regen. Put on a spear and some +effective arc effects and go wading through a crowd of skaven. Then do the same thing, but using Glaive instead, and without the effect arc. Your block will break substantially faster as many more attacks will be coming in from outside your effective arc and so drain your stamina faster.
Also worth noting is that if you’re using the Saltzpyre career that makes blocking light attacks cost no stamina, then it only applies while within the effective block and push angle. Increasing that angle will increase the number of attacks that are blocked for zero cost.
I haven’t tested it yet, but I assume that “timed parries cost zero stamina” trait also likely needs to come from that effective arc.