1/15 chance to increase damage by 0.5 for the current room each time an enemy dies.
Samson's Lock can activate up to 10 times per room.
The activation chance depends on the Luck stat and goes up to 100% with 10 Luck.
Synergies
Lusty Blood: Both the item and the trinket stack, granting 1 damage per kill. With 100% proc rate on the Lock, Isaac may gain up to 10 damage after 10 kills.
Rock Bottom: Preserves the highest bonus earned between rooms.
Golden Trinket / Mom's Box: At base luck, having either increases the chance to 13% and having both to 25%.
Interactions
Ace Cards: Enemies that are turned into pickups don’t count towards the Samson's Lock counter.
E. Coli / Flush!: Enemies that are turned into poop don't count towards the Samson's Lock counter.
Freeze Effects: Enemies that are frozen don't count towards the Samson's Lock counter.
Because of this interaction, when Isaac has Uranus, only enemies killed by icicles from statues, bombs, familiars and other sources will increment Samson's Lock's counter, thus rendering Samson's Lock quite difficult to use.
According to the Bible, Samson was given supernatural strength by God to fight his enemies, the Philistines, and perform heroic acts; but when he was tricked into allowing his hair to be cut by a Philistine woman named Delilah, Samson was powerless.
A lock of hair is a piece(s) of hair which has been cut from a human head, typically being held together in some manner.
This trinket is a reference to the effect of Bloody Lust (Samson's starting item) in the original game, which was later changed in Rebirth and added in Afterbirth as the item Lusty Blood.
This trinket has the same flavor text as Red Patch.
Lusty Blood: Both the item and the trinket stack, granting 1 damage per kill. With 100% proc rate on the Lock, Isaac may gain up to 10 damage after 10 kills.
Rock Bottom: Preserves the highest bonus earned between rooms.
Golden Trinket / 
Mom's Box: At base luck, having either increases the chance to 13% and having both to 25%.
E. Coli / 
Flush!: Enemies that are turned into poop don't count towards the Samson's Lock counter.
Uranus, only enemies killed by icicles from statues, bombs, familiars and other sources will increment Samson's Lock's counter, thus rendering Samson's Lock quite difficult to use.
Bloody Lust (Samson's starting item) in the original game, which was later changed in Rebirth and added in Afterbirth as the item 
Red Patch.
