What's going on with curve in AdEnv

Software Development
2

I guess I can answer my own question, in part, by looking at the code. The key section is here:

    {
        curve_x_ += (curve_scalar_ / time_samps);
        val = beg + inc * (1.0f - EXPF(curve_x_));
        if(val != val)
            val = 0.0f; // NaN check
    }

Where EXPF is a function that basically computes x^10:

inline float expf_fast(float x)
{
    x = 1.0f + x / 1024.0f;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    x *= x;
    return x;
}

…but I don’t really know why it does that, or why it behaves as strangely as it does. I watched a video on envelopes (found the link in an earlier discussion here) and while I can follow the math and the code in general, I’m not quite sure what’s going on in this case. Anyone have a clearer explanation, or a suggestion to improve AdEnv?