Rinse cucumbers and pat dry. Cut crosswise into pieces about 4 inches long. Cut each piece in half lengthwise.
On a work surface, place a piece of cucumber (or several) cut side down. Lay the blade of a large knife flat on top the cucumber and smash down lightly with your other hand. The skin will begin to crack, the flesh will break down and the seeds will separate. Repeat until the whole piece is smashed. Break or slice diagonally into bite-size pieces, leaving the seeds behind.
Place the cucumber pieces in a strainer and toss with a big pinch of salt and a big pinch of sugar. Place a plastic bag filled with ice on top of the cucumbers to serve as a weight and place the strainer over a bowl. Let drain 15 to 30 minutes on the counter, or in the refrigerator until ready to serve, up to 4 hours.
Make the dressing: In a small bowl, combine salt, sugar and rice vinegar. Stir until salt and sugar are dissolved. Stir in sesame oil and soy sauce.
When ready to serve, shake cucumbers well to drain off any remaining liquid and transfer to a serving bowl. Drizzle with olive oil and toss. Add half the dressing, half the garlic and the red pepper flakes to taste, and toss. Keep adding dressing until cucumbers are well coated but not drowned. Taste and add more pepper flakes and garlic if needed. Serve immediately, garnished with cilantro and sesame seeds.