What does helping hand do on Pokemon Diamond?

Helping Hand in Pokemon Diamond only has an effect in a Double Battle. Otherwise, it does nothing.

When you are in a Double Battle, like in the Battle Tower, and you had two Pokemon - Monferno and Plusle, while you were facing a Baltoy and Cherrim. Your Plusle would first use Helping Hand. Then your Monferno would attack using Grass Knot, and would knock out the opposing Baltoy in one hit. If Grass Knot used against a Baltoy had a power of 50 x 2 (for weakness), you would normally do 100 damage, not likely knocking out Baltoy. But, with the bonus of Helping Hand, you would actually have a power of 150, probably knocking out the Baltoy.

To put a long story short, whether you second Pokemon's move is Physical or Special,

the power of that move would be multiplied by .5. So, Helping Hand really can help you, if you use your brain first.