Understanding the models
This may get pretty hairy, as this is actually a simplified version of differential equations.
I don't expect everyone to understand this first bit, and so I recommend the "Tweaking your settings" if you just want to tweak the mod.
Here are some graphic representations of the models (Higher resolution can be found in images)
So what do those extremely weird and names and lines mean?
Heart rate models
This is a picture showing how fast your heart rate will change, based on what your heart rate is
X-axis (Horizontal): on the x-axis you see the difference between your current heart rate, and your heart rate cap. This means if your heart rate is at 60, and you're sprinting, the x-value will be the difference between your sprint-cap and your heart rate in this case (and with default values) 130.
Please note, that the x-axis is offset by 60, so 130 x will mean you're on the "Default sprint cap" mark.
Of course if your heart rate is higher than your heart rate cap, the values will be negative.
Y-axis (Vertical): Here you see the rate at which your heart rate will change. The higher up you get on this axis, the faster your heart rate will change. This means that your x value will change, the more your heart rate changes, since the x-value is the difference between your heart rate, and your heart rate cap.
The function graphs show how your y-value changes dependent on your x-value, and what models correspond to what graph.
Long Term Stamina
X-axis (Horizontal): This shows your absolute heart rate, simple as that
Y-axis (Vertical): This shows how much your LTS will change per second, be it positive or negative.
The functions here show how you y-value changes when your x-value changes, depending on which model you use.
As you see, the Threshold is shown on the graph. This is because when you change the threshold, you will "push" or "pull" the functions in the direction you're changing it in, so a heart rate below the threshold will correspond to a positive change, and a heart rate above the threshold will correspond to a negative change.
A documentation of the pure functions (Tiny bit simplified)
HeartRateMultiplier*(0.2 + (1/3200)*(HeartRateCap - HeartRate - 2*Threshold)^2)
HeartRateMultiplier*(0.2 + (1/40)*(HeartRateCap - HeartRate - 2*Threshold))
LTSMultiplier*((HeartRate - Threshold)^2)*4.5/20280
LTSMultiplier*(HeartRate - Threshold)*3.15/156