Wrap each cucumber in plastic wrap to minimize splattering. Place on a work surface and pound with a flat object (like a meat pounder) until cucumbers crack and are slightly flattened. Remove from plastic. Cut cucumbers in half; halve each side lengthwise. Slice into 1- to 1 1/2 -inch wide slices. Transfer to a strainer set over a bowl.
Sprinkle cucumber with sugar and salt; mix until well combined. Refrigerate bowl and strainer for 30 to 60 minutes so cucumber pieces can drain into the bowl.
Place garlic, seasoned rice vinegar, soy sauce, sesame oil, and red pepper flakes in a large bowl. Whisk together thoroughly.
Transfer drained cucumbers into bowl with dressing. Toss to coat. Cover with plastic wrap; refrigerate for about 30 minutes to allow flavors to mingle.
Place cucumbers in serving bowl and sprinkle with sesame seeds.