Sukuna's Fingers are Cursed Objects that contains fragments of Ryomen Sukuna's Soul.
Eating a finger gives several buffs and one debuff such as: The Ability to use Sukuna's Cursed Technique alongside other Cursed Techniques, Fall Damage Cancel, Air Jumping, RCT and Domain Expansion even if you don't have the advancement, Curse User Alignment that changes what mobs will be immediately hostile and a FATAL weakness to Angel's Jacob's Ladder. While having the Sukuna Effect, you cannot earn achievements as it is theoretically Sukuna killing and earning stuff for you, not yourself. If you're going to eat Sukuna's fingers using with any Cursed Techniques, it is recommended to earn the necessary achievements you might need before doing so.
As of update 39, Sukuna Finger Vessels received a buff that halves the Curse Power consumption on techniques.
As of update 40, Sukuna Finger Vessels received a new passive that automatically slashes projectiles if they are shifting.
Consuming
Itadori's Cursed Technique:
Consuming a finger as Itadori allows you to be able to switch from Itadori and Sukuna that has a 5 minute duration time. Eating 10 fingers in a short period of time causes Sukuna to forcefully take over for 5 minutes.
Megumi's Cursed Technique:
Consuming a finger as Megumi allows you to use the Ten Shadows and Sukuna's Cursed Technique at the same time, unlike Itadori the effect has no time limit.
Sukuna's Cursed Technique:
Consuming a finger as Sukuna has no special effects other than buffing you and halving your Curse Power consumption, like Megumi the effect has no time limit.
Other Techniques:
Consuming a finger while using other Cursed Techniques will usually result in you instantly dying, however having the comedian effect applied to you will let you eat a finger as any Cursed Technique. In order to eat Sukuna's Fingers with other techniques, you must be a Cursed Spirit, but the buffs you receive are lesser and you do not get his Cursed Technique.
Other Buffs :
The image provided below are the buffs of each stage of eating Sukuna's Finger.
Obtainment
There are 2 ways to obtain Sukuna's Fingers: 100% Drop Chance from Cursed Spirit: Insect as long as you hit him and 1% drop chance from Stevenson Screens.