I definitely don't know much about finance, but a short is essentially being obliged to sell a position at a future date, right? You match your short position against the fraction of your ETF which is in TSLA, such that you are obliged to sell in the future exactly as many TSLA shares as you indirectly own through the ETF. This way you do not need cash on hand, because you can sell a portion of your ETF to pay off the short.
Theoretically it's not risky because in the scenario that the short becomes expensive, TSLA has gone up the and in turn TSLA has made your ETF appreciate the same amount that you owe due to the short, and vice versa if it goes down.
Theoretically it's not risky because in the scenario that the short becomes expensive, TSLA has gone up the and in turn TSLA has made your ETF appreciate the same amount that you owe due to the short, and vice versa if it goes down.