Maxim, 

The references you sent are extremely interesting. I hope this literature ensemble will answer the question. 

I agree . The references are interesting and good. All the best

I think the short answer is that nobody knows.  However, it probably has to do with training the autonomic nervous system and the sensitivity of baroreceptors.  Other activities such as slow breathing and meditation are also very effective ways of reducing BP and I suspect that muscle contraction, especially if it is rhythmic as when doing one contraction every 10 seconds during a set.  There is some (limited) evidence that such muscle contractions can entrain HRV (Lehrer) in a similar way to slow breathing