With regard to the clay, they do say to keep turning it over every now and again to find a clean bit but and this is what I dont understand. The particles that come off are debris and will stick to the clay. The clay is not a huge piece but only about 3" square and it is supposed to be enough to do a complete car so how the hell are you supposed to find a clean bit!!!!!!!!!!!!!!!!!
With regard to electric polishers or attachments for an angle grinder or indeed a power drill.
Again as with the clay, be very careful about the amount of pressure used as you will invariably end up with swirls if not 100% competent in what you're doing and if the vehicle happens to be black then they will look 100% worse in the sunlight.
If you watch some vids of proffesional detailers at work ( plenty on you tube) you will see how many different processes they actually go through and believe me it all takes lots of time.