Detuning is done for many reasons. Simply put it is using various means to lower the power output of a motor. One example is a motor making 160hp may need to be detuned through PCM adjustments to 150hp to meet lower insurance premium levels in another country. Or, in racing series, you might detune a motor to be legal for a certain class that has a maximum horsepower restriction.
Push rod engine is another way of saying overhead valve (OHV). There is a lot to get into on this subject so I'd suggest starting on Wikipedia, reading about overhead cams vs overhead valves. If you'd rather the short version, overhead valve generally indicates cam in block design, and benefits include reliability, typically better low-end torque, and lower cost and complexity. Overhead cam engines generally are capable of turning a bit higher RPMs and make more top end power. Of course there are exceptions to the above arguments but when comparing the 4.0 OHV to the 4.0 SOHC, consider the SOHC to have more top end power at the expense of reliability (mainly timing chain and guide wear).
