As The Forgotten, the floating head will only fire tears when the Soul is active.
The floating head will not fire tears as Tainted Samson while he is enraged.
Each wave of Boss Rush fully recharges the item, allowing many heads to be spawned.
Only deaths which end runs (i.e. Isaac is not revived by a revival item such as 1up!) count towards unlocking the Scissors.
Synergies
Book of Virtues: Spawns red wisps in the middle ring. The wisps fire 2-3 tears at a time, which are inaccurate, travel in an arc, and have variable range.
Epic Fetus / Marked: Blood fountain from the body will aim at the crosshair.
The Pinking Shears: If Scissors and Pinking Shears are used together, control is given back to an invisible head with only a shadow visible as both the head and body are mobilized to attack.
BFFS!: The loose head won't become larger, but will deal double damage.
C Section: C Section tears are fired from the body instead of the head. This makes the head do nothing, but the item still provides a benefit as the body will fire both the C Section tears and the blood fountain from Scissors.
3DS The icon of Scissors as seen on the lower 3DS screen
Trivia
This item is very likely a reference to the English idiom "running with scissors", which means to "willfully act in a reckless, foolish, and/or dangerous manner", alluding to the literal act of running with a pair of scissors in one's hand, which can cause severe injury, thus the item's effect and unlock condition.
In original flash version of The Binding of Isaac, dying 100 times unlocked The Pinking Shears.