well it's a complicated procedure...even the so called mud which isn't just dirt and water at all but was long ago is a pretty high tech substance with a mixture of synthetic and organic fluids and solids that they can/have to change the weight and viscosity of over time to match conditions in the well. They also have to back off the tens of thousands of hydraulic horsepower pumping they have on site at times in order to get accurate pressure readings in the BOP to see what's going on there. yeah the flow rate needs to drop here before they can start pumping concrete but like putting your finger over a water hose a partial blockage decreases the flow rate but increases the pressure so you can't judge success by looking at what's coming out of the BOP. I'm not really following it...I sure don't have the connection bandwidth to watch the live streaming video and most of what I have read up on was an attempt to understand what went wrong. But I've learned enough to know that the technology and techniques are pretty impressive and ingenious and the work is very specialized with a single subsidiary of Haliburton in charge of just pumping and analyzing the drilling mud (before the blowout that is and probably now too). You could spend years just learning all about drilling mud and the 20 or so different functions it serves. Anyway I hope it works and the sooner the better.