The indirect back pipe works as you described. It basically makes the exhaust heat go down one half of the pipe, make a U turn at the bottom then go all the way back up to then go out. It is a simple addition to the stove that makes a big difference in how it works with coal. It raises the thermal efficiency of the stove by at least 15%, possibly more; during its optimal operating range. That adds up to a significant savings in coal consumption.