Once you have the Strength HM, you can teach it to a Pokémon that can learn HMs by talking to the Move Maniac in any Pokémon Center.
To use Strength, select it from the move list of a Pokémon that has it equipped, and then select the rock you want to move.
Make sure the rock is marked with a yellow "X".
Not all rocks can be moved with the strength hidden move.
Strength can be used both in battle and outside of battle. In battle, it deals damage to the opponent and has a 50% chance of lowering their defense. Outside of battle, it can be used to move boulders or other objects that obstruct your path.
Here's a step-by-step guide on how to use Strength in Pokémon Diamond:
1. Obtain the Strength HM from the Rock Climb tutor in Oreburgh City.
2. Teach Strength to a Pokémon by talking to the Move Maniac in any Pokémon Center.
3. Select Strength from the move list of a Pokémon that has it equipped in battle or move selection menu while on the world map.
4. Select the rock you want to move outside of battle.
5. Your Pokémon will use Strength to move the rock, potentially revealing new paths.
Remember that Strength is a physical attack, so its damage is affected by the user's Attack stat and the target's Defense stat.