You have to meet other people and take their flags in the Underground.
You use Strength to roll the movable boulders.
you can use the hm strength you can smash rocks with rock smash
you have to obtain the HM strengh and teach it to a Pokemon in your party. after that just stand in front of the rock and press the B button.
Get strength by finding the wardens teeth in the safari zone and he will give it to you.
you have to get fifty flags apparently to remove all the boulders.
you are able to remove all boulders. I say its worth it
You have to meet other people and take their flags in the Underground.
those are boulders that have to be pushed around by using strength
you mest get the hm strength.
The HM strength
You teach a Pokemon strength.
steal other players flags, when you get enough you can remove boulders
The move in Pokémon Diamond that allows you to move boulders that are in the way as you're traveling the Sinnoh region is called Strength. Strength is a Hidden Machine that you can find in the top floor of the Lost Tower.
You use Strength to roll the movable boulders.
you can use the hm strength you can smash rocks with rock smash
After you beat the sixth gym leader, you are allowed to use strength which moves the boulders.