Once the trigger price is hit, a market order is created.
Bitcoin is currently trading at 55k. Alice decides to short 0.1 bitcoin once it trades under 51k. Alice sets a stop market sell order for 0.1 bitcoin at 51k. As soon as it trades under 51k the keeper-bots execute a market sell. Her execution price can be below 51k. Should the market shift rapidly, it can be substantially less than 51k USD.
Sushi is currently trading at 20 USD. Bob decides to buy sushi once sushi trades over 25 USD. He sets a stop market buy order with a trigger price at 25 USD. Once sushi trades over 25 USD, the keeper-bots will execute a market buy order on behalf of Bob. His execution price can be over 25 USD. Should the market shift rapidly, it can be substantially more than 25 USD.