Are you asking how did you tear your ACL?
Or about the mechanism often related to ACL tears?
The ACL is the anterior cruciate ligament, located in the knee, helping to stabilize the tibia (bone of the lower leg).
And it is stress on this knee joint - particularly direct impact on the knee from the front or the outside -- which causes injury to this ligament.
In particular, females that participate in gymnastics, soccer, and basketball have much higher rates than average for ACL tears just because of the types of movements in those sports.
The typical scenario is a person who is running -- then plants their foot and suddenly slows down to change direction, or does the same and tries to pivot on their leg -- and puts too much stress on the knee while it is outstreched and attempting to turn.
Dribbling a soccer ball and "cutting" to the left or right with a sudden movement is a typical scenario for injury.
Also direct injury - such as a player tackling you from the outside of the knee, causing the knee to buckle inwards is another mechanism for injury (less common though, as most ACL tears come from non-contact injuries)
There are a bunch of theories, but no one really knows:
- men have stronger thighs and leg muscles, and are better able to stabilize the knee against injury
- females tend to put more inward stress on their knee to suddenly change direction
- estrogen may exert a slight weakening effect on the ligaments
But the one thing that is known - when comparing soccer and basketball players male vs. female -- females tend to have ACL tears about 2-3 times the rate of their male counterparts.
Well, you can do anything to injure yourself if you want.
One way might be to plant your foot and have the knee hit directly from the outside (laterally), making the knee buckle inwards (medially). This would put stress on the ACL.