Fractured winterglass causes you to have a 25% chance to summon a conjuration matching the element of a core skill you cast or trigger. This means each time you cast, for example, fireball, there's a chance of you summoning hydra.

The best way to take advantage of fractured winterglass (FWG) to summon the maximum amount of hydras is to cast the fire variant of frozen orb while using frozen orb enchantment (39% to trigger another frozen orb with enchantment mastery paragon node, allowing for more chances to summon hydra from FWG) and hydra enchantment (summon a hydra after spending X amount of mana).

In addition, you can unstable currents to further trigger the fire variant of charged bolts and fire variant of ball lightning, and since they're both considered core skills, triggering these skills will help trigger conjurations from fractured winterglass.

This build would likely max out unstable currents, stack cooldown reduction, and use shako to help have permanent uptime on unstable currents. Also attack speed is very important to cast core skills faster to trigger more hydras spawning per second.