Have used this gem all the time since i saw a build that used it. Now i looked at my gems and was wondering whats the use for it.

Rarity: Gem
Fortify Support
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Attack, Support, Melee
Level: 20 (Max)
Mana Multiplier: 110%
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Requirements:
Level: 70
Str: 111
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Supports melee attack skills that are not triggered.
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Supported Skills grant Fortify on Melee hit
Supported Skills have 25% increased Fortify duration
Supported Skills deal 44% increased Melee Physical Damage
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This is a Support Gem. It does not grant a bonus to your character, but to skills in sockets connected to it. Place into an item socket connected to a socket containing the Active Skill Gem you wish to augment. Right click to remove from a socket.

You proc it with shield charge.

Fortify is a buff that grants the player 20% reduced damage taken from hits for 4 seconds.

in hardcore a lot of players link fortify to their main melee setup to its up 100% of the time..

In softcore, every build using shield charge / whirling blades / leap slam... should be using fortify (melee movement skill + faster attacks + fortify)