A simple looking metal yo-yo on a thin and flexible metal cord.  On the side of the finger loop there is a hidden switch.

Pressing the switch cause the yo-yo to reveal it's hidden blades.

Proficiency with a Flail allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, As a bonus action or a called preparation a player can hit the switch to release the blades. The player attacks using yo-yo tricks. The player can attempt an Around the World to hit multiple targets. If the player makes a dexterity check with a DC of 15 then they will roll one attack and one damage roll that will be applied to all targets. If the player fails the check then they take damage as they mess up the trick., Damage, Combat, Sap