It was new to me as well but luckily I found a solution: HOW TO SOLVE DREAD’S CAPTCHA – GUIDE - It still took me around 10 more attempts, before I succeeded in solving it.
LOL one of the hardest logic I have ever learnt. The blogpost you gave did not help me at all and then I went to youtube to see how it works. But still it took me more than 10 minutes to figure out and to solve my first Dread captcha :-D