The connecting rod connects the pistons to the crankshaft. It converts the linear motion of the pistons to the rotary motion of the crankshaft. On every stroke, the connecting rod is stretched and compressed. This pressure, plus other factors, can cause the connecting rod to break. The broken rod can go through the engine block completely, ruining the engine–a condition known as “throwing a rod.”
Fatigue is the main cause of broken connecting rods–especially in older engines. The constant compression during the power stroke and stretching during the exhaust stroke, over thousands of times a minute, eventually wears the metal out and it becomes brittle and finally breaks. If the oil is low or dirty it can speed up this process. Running the engine hot can also speed up the process. Sometimes a fairly new engine can have fatigued connecting rods if it is a rebuilt engine and the mechanic used cheap parts or the wrong parts for the engine.