From Brotato Wiki
+8 Ranged Damage
+8 Elemental Damage
Your Max HP is capped at its current value
Item
Handcuffs is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Unique
Handcuffs is a unique item. Only 1 of each unique item can be equipped, and it will then no longer be offered in the Shop, and no longer drop from loot crates.
Handcuffs
Handcuffs provide a large amount of damage for a small cost. But it comes with the downside of locking your Max HP when you buy it. You can still increase or decrease your Max HP Stat, but when you enter the game, your health bar can't be higher than your Max Health cap. For example, if you buy handcuffs while you have 50 Max HP, your Max HP can never exceed 50. However, you can still buy Max HP-reducing items and have less than 50 Max HP. The lock doesn't prevent you from getting lower.
Even if you buy Handcuffs, HP-Scaling Items and Weapons like 
Alien Eyes, 
Potato Thrower, and 
Chopper can still scale beyond the Max HP cap because they scale with the stat itself. The cap only affects your HP Bar.
Handcuffs can be a powerful pickup late in the game if you've already gotten the Max HP you wanted for the wave 20 boss fight.