With a sharp smack or whack (onomatopoeic); used to describe striking something forcefully and decisively.
彼は犯人の肩をバシッと一発叩いて念を押した。
He gave the culprit a sharp slap on the shoulder to make a point.
布の埃をバシッと叩いて落とした。
I gave the cloth a good whack to knock off the dust.
風船をバシッと割って驚かせた。
I popped the balloon with a sharp smack to surprise them.
Directly; decisively — used when answering, deciding, or refusing in a clear, decisive manner.
面接で彼はバシッと自分の強みを述べた。
In the interview he spoke about his strengths in a clear, decisive way.
冗談を言う同僚に対して、上司はバシッと注意した。
The boss sharply reprimanded the joking colleague.