Here's why:
* "Seen" is the past participle of the verb "to see," and it's used in perfect tenses (e.g., "I have seen," "I had seen").
* "Saw" is the past tense of "to see" and is used for actions completed in the past.
In the sentence "I just saw you do it," you are describing a recent action ("just saw"), so the past tense "saw" is the correct choice.