If it is the 5:2 diet, It does seem to be pretty popular right now, and I am indeed trying it.
I was originally made aware of the diet in 2011 watching a Horizon Documentary on it by Dr. Michael Moseley. The basic premise is that of calorie restriction ; Tests in rats and then later in small trials of humans showed that a 2 day fast, where you took in a maximum of 600 KCal if an adult male, 500Kcal a day for adult women - and then ate pretty much whatever you wanted for the other 5 days resulted in a steady weight loss.
Part of the success was that, because you only perceive yourself as being on a diet for 2 days a week, people were more compliant with the schedule, and just the knowledge that they could eat whatever they wanted on the other days acted as a kind of mental reassurance - but researchers found that people were actually quite sensible on the other 5 days.
so the proposed diet plan has been refined somewhat now - 2 days calories restriction, maximum 600 KCal/ $00 KCal, other 5 days eat normally but sensibly/ healthily.
There wre some possibly interesting side effects as well, but these are much less well established and much much more work needs to be done - but some studies appear to show that rats for instance that have a calories restricted diet actually live longer than their counterparts that eat pretty much whatever they want. There is a proposed mechanism for this involving a protein called Insulin-like Growth Factor- 1 (IGF-1). Lower levels appeared to equate to longer lifespan - and in the Horizon documentary, Moseley monitored his IGF-1 levels, which did seem to come down over time following this diet plan.
I was initially quite keen on this diet, the attraction being it was relatively easy to do, with the reassurance that you could eat whatever you wanted for the other 5 days, and indded initial weight loss was quite promising. But - I have now decided that for more marked and long term benefits, I really need to alter my consumption of food and drink, and aim for just eating and drinking much less, 7 days a week, and eating more vegetables and fruit, and less meat and processed food and snacks.
This approach actually seems more effective. Boring, and requites self discipline, but I have found it more effective :)