Quarter the cucumbers lengthwise. Put the slices on a cutting board, skin-side-up. Use the flat side of your knife to lightly smash along the length, then cut the cucumbers into 3/4-inch pieces. Toss the cucumbers with 1/2 teaspoon each salt and sugar, then drain in a colander for 20 minutes.
Whisk together the vinegar, soy sauce, sesame oil, garlic and remaining 1/2 teaspoon sugar. Add the cucumbers and toss to coat. Sprinkle with crushed red pepper, if using.