downunder

A 2 stroke conrod basically only ever experiences compressive forces and never tensional forces. Tensional forces are only felt in 4 stroke engines at the end of the exhaust stroke and during the first half of the intake stroke. These compressive forces come from two things. First (and most obvious) is the pressure on the piston from combustion. Maybe less obvious is the fact that the piston has to change it's speed from zero at the top and bottom of the stroke to a maximum speed roughly halfway up the stroke. That means the piston has to be accelerated. In a .21 engine at 30,000 revs that means the piston goes from a standstill to roughly 5000 feet/minute (~60 mph) in 1/2000 of a second. The piston gets accelerated (by the rod) at around 10,000 G's. On the powerstroke the combustion pressure does all the accelerating so it's compressing the rod.

Now over-rev to 40,000 and the piston acceleration increases to over 18,000 G's which is almost double what it had at 30,000. Just where the rod will break depends on the design and it's weakest point. Because it's at an angle to the crankpin at maximum load it's bending first in one direction then the other so it's being flexed at a very high rate and aluminium fatigues fairly quickly.