An example would be the arms that begin the movement of a jump (i.e. momentum) before transferring the energy through the chest and abdominal area, ultimately delivered to the legs to enhance a jump upwards. It is why you can jump higher with your arms than without.
Unfortunately, several exercises like the leg press, do not involve the trunk musculature as much as an axially loaded (fancy science word for something oriented vertically) exercise like squats. Recently, even fitness industry experts like Mike Boyle have abandoned squatting for single leg movements because they involve more lower body activation.
I agree that a single leg exercise like lunge has more relative leg activation due to less trunk stimulus (i.e. the weight on the shoulders is generally less). Unfortunately, several studies and my own coaching experience has shown that this trunk strength is more of a deciding factor in elite athletes’ performance than leg strength.