Smash the cucumbers: Place a cucumber on a cutting board. Using a rolling pin, heavy knife blade, or your fist, smash the cucumber until it cracks open but stays in large pieces. Repeat with remaining cucumbers. Cut crosswise into 1-inch chunks.
Salt and rest: Transfer cucumber chunks to a colander set over a bowl. Sprinkle with 1 teaspoon salt, toss, and let rest for 10 minutes. This draws out excess water.
Make the dressing: In a small bowl, whisk together soy sauce, rice vinegar, sesame oil, sugar, garlic, and chili flakes.
Combine: After 10 minutes, gently squeeze the cucumber chunks to remove excess liquid (don't crush them). Transfer to a serving bowl. Pour dressing over and toss gently. Sprinkle with sesame seeds.
Serve immediately or refrigerate for later. The flavors meld as it sits.