Slice the cucumbers into thin, uniform rounds using a sharp knife and a steady hand. Place them in a mixing bowl.
In a small bowl, whisk together rice vinegar, sesame oil, chili flakes, salt, and sugar until the sugar dissolves completely. You'll smell a fragrant, tangy aroma as you mix.
Pour the dressing over the sliced cucumbers, ensuring all pieces are coated evenly. Toss gently with a spoon to combine everything thoroughly.
Let the salad sit for at least 5 minutes to allow the flavors to meld and the cucumbers to absorb the dressing. The cucumbers will become slightly translucent and glisten as they soak up the flavors.
Give the salad a final toss to redistribute the dressing and check seasoning, adjusting with more chili flakes or salt if desired.
Serve the spicy cucumber salad immediately in small bowls, garnished with a sprinkle of sesame seeds or chopped fresh herbs if you like. The salad should be crunchy, vibrant, with a zingy, fiery kick.