This is the 949th most frequent Japanese word.
ッ
The small "ッ" (sokuon) indicates a doubled consonant or a brief pause in pronunciation.
The katakana 'ッ' in 'サッと' is used to create a sense of abruptness or swiftness in modifying the adjective.
彼の行動はサッと変わった。
His actions changed swiftly.
The 'ッ' in 'ハッキリ' strengthens the sound, giving emphasis to the clarity of speech.
もっとハッキリ話してください。
Please speak more clearly.
The 'ッ' in 'バッと' conveys a sudden, rapid action, emphasizing the immediacy of starting.