Cover cucumbers with a towel, and evenly pound with a kitchen mallet, until cucumbers crack and are slightly flattened. Cut into 1-inch pieces, and break apart any large pieces where cracked.
Transfer pieces into bowl, and sprinkle with salt and sugar. Mix well and refrigerate for 30-60 minutes.
In a large bowl combine garlic, rice vinegar, soy sauce, sesame oil, and red pepper flakes; whisk together thoroughly. Transfer drained cucumbers into the bowl with dressing and toss to coat. Refrigerate 30 minutes.
Roast sesame seeds at 350 degrees for 10 minutes on a sheet pan, stirring occasionally. Place cucumbers in a serving bowl, and sprinkle with sesame seeds as desired.