Thread.sleep() will prevent Java memory inconsistency when testing volatile

Question

I'm testing the memory consistency function of the volatile keyword.

This is code that does NOT use volatile, the program will be stuck as expected.

package cn.demojie.singleton;

public class VolatileTest2 {
  private static boolean flag = false;

  public static void main(String[] args) throws Exception {
    new Thread(
            () -> {
              while (!flag) {
                /* Uncomment it, the program won't get stuck anymore
                try {
                  Thread.sleep(0);
                } catch (InterruptedException e) {
                  e.printStackTrace();
                }
                */
              }
            })
        .start();

    Thread.sleep(50);
    flag = true;
  }
}

But when I add Thread.sleep() like the comments above, the program won't get stuck anymore.

It seems the memory consistency is in effect. Why?

System.out.println() also works as the Thread.sleep()