When a slider-crank mechanism approaches dead center, as shown in this animation when red links becomes straight, there is a rapid increase in the ratio of resistance Q to vertical driving force P.
This ratio can be expressed as
Q/P = 0.5/tan(alfa)
Where alfa approaches zero when red links becomes almost a straight line.
This means that very great resistance can be overcomed by small force P. The toggle mechanism is used in many forms in practice. For example in the stone crushers as shown this flash animation, or presses, pneumatic riveters, friction clutches, etc.