As Simon says it remaps the accelerator and increases the regen available on both D and B modes. Nissan's research found a 2% increase in range - frankly not worth it for the feeling of dullness that it induces.
Are you sure it increases the regen ?
I agree with @Hermit Dave - adding Eco to D takes the regen somewhere between "non-Eco" D and B, and adding Eco to B seems to make the regen come in earlier and harder. But I don't think that it increases the absolute limit - if I use B at 50+ adding Eco seems to make no difference to the regen effort.The regen braking for eco is greater than D and less than B.