The Strength Belt is a great item if you have a powerful Pokémon that can hit hard but can't hit on its own. In Pokémon Sword and Shield, you can equip it to your Pokémon. This prevents them from falling after a hit, allowing them to keep a point in their health pool even if they should have fallen. However, this only works if they have full health at the time they are hit. So you want to give them to a Pokémon with low defense.
To add this item to your collection of useful tools, you must beat Gym Leader Third. After that, you gain access to the largest city in the north, Hammerlocke. Otherwise, it's stuck until you've completed the first three gyms.
As you enter town, head to the house to the left of the Pokémon Center. It should be a two story house, and when you walk in there's a man in a blue gi with a red headband on his head. Talk to him and accept his challenge in battle
It should be a pretty quick fight. He has a level two Doudouvet, which you could probably take out with one hit. It didn't matter if you could get away with it on the first try, it made the comeback extremely difficult. After you eliminate the Doudouvet in the fight, Mr. Focus will want to talk to you again.
For your hard work, he gives you the Strength Belt, which you observed during his battle. Make sure to equip a Pokémon with a large amount of attack, which usually doesn't last long in battles. The item disappears after its use, however.
Once you've completed the game, you can purchase it at Wyndon's Battle Tower using The Battle Points, which you can earn by winning The Tower Battles. You may want to keep the item until you can get more of it after beating Sword and Shield.
