Slap

Without saying a word, Rachel shifted her gaze back to her plaything, breathing heavily through his nose, staring pleadingly at his torturer much to her amusement.

She adjusted her awkward position on the stool in a fruitless attempt to sit more comfortably, momentarily closing her legs before spreading them wide again, casually disregarding the exposed girl-parts unveiled perfectly in line with Paul's gaze, not that he had much time to admire this exhibition.

Rachel smirked deviously, then tightened the grip around her still encircled new toys and gently pulled them slightly upward slowly stretching the orbs through the scrotum to their maximum distance, carefully stopping the moment Paul started to wince.
Concentrating her stare on the protruding boy-parts, swollen and ****, she raised her slender free hand only slightly then unleashed a savage barrage of quick repeated palm slaps on her miserable classmate's aching nuts.
Her girlish hand became a blur as it swiftly but accurately swatted the swelling testicles in an excruciating avalanche of merciless strikes.

Paul's whined softly through his gag as each steady thump and whack was not terribly painful by itself, but the speed of the young girl's delivery multiplied them into a rapidly growing explosion of misery, causing the pitiful young man to clamor in an increasingly louder shrill, which soon drowned out the regular 'splat' sounds resonating off the basement walls.
He desperately tried to reach towards his assaulted manhood, eyes stretched wide, causing Rachel to cackle brightly at the sudden pathetic climax to the progressively growing response of male anguish.

The outburst of joyous laughter broke her mischievous concentration and her dainty palm slipped off the bloated eggs, battered and sore, slowly regaining their original shape above the circle formed by the girl's ensnaring hand.

When Rachel's laughter subsided, she took a moment to catch her breath then looked up at her older sister, smiling joyously.