how san disc works?

There's no card named "San" in my database, so I can't rule on that specific card. If you mean the discard mechanic in general, here's how it works:

• Discard = moving a card from your hand directly to your trash — it never touches the board. Cards on the battlefield can't be "discarded"; they're Killed (Rule 422.1).
• When discard is a cost ("as an additional cost, discard 1"), you must have enough cards to pay it. When it's an effect ("discard 2"), you discard as many as you can and the rest of the effect still resolves (Rules 422.3–422.4).
• "When I am discarded" triggers (like Flame Chompers) fire after the discard happens, and the discard is usually not a cost — it happens during resolution, so your opponent reacts before seeing what you pitch (FAQ #2724, #6551).

If "San" is a specific card (possibly from a newer set), give me the full card name and I'll pull it up.