When done correctly or when it is done correctly?

Hi. Please help. I see subordinate clauses like "when it is done correctly" shorten to "when done correctly." I think we can use short forms like "when done correctly" as long as the referent is clear, like the example sentence below, where the referent is present clearly: When that activity is done correctly.

This activity is really good. When done correctly, I think it can help students greatly.

But when the referent is unclear, I believe we should avoid using short forms like "when done correctly." What is your opinion? Thank you in advance for your help.