Ok so let me start this post by just defining the strength curve. The strength curve is a graphical representation of how the body generates force through a specific range of motion. Different exercises will train the muscles through different parts of the strength curve (low, mid, upper). Now, to thoroughly hit all fibres and really get the best from an exercise you need to overload the strength curve. By tweaking an exercise that let's say normally trains the muscle through the low range of the strength curve, you can work the same muscle through the mid-upper range. Let me further demonstrate this with an example aimed at working the biceps thoroughly...
1A: ez bar preacher curl holding onto straps
Strength curve target: mid-upper range
1B: ez bar preacher curl
Strength curve target: low range
By holding onto straps and moving the weight of the ez bar outside the centre of gravity you'll work the biceps through a different range of the strength curve. Now that's how you really push 1 exercise to its limit!