You can sell stock that you don't own by 'shorting', having a...

You can sell stock that you don't own by 'shorting', having a counter party 'loan' you their stock for a fee. If you can create downward momentum, possibly including triggering stop losses, you can by the stock back at the lower price to close out your agreement with the counter party. Sell high, buy low.

If you are going to take a position, or the party you are acting on behalf of is going to take a position, the now lower entry price may make that position cheaper.

I think this is what zmickeyzee1 may have been referring to.