That's what I feared - fixv() and floatv() are really hard to explain and to understand because they go against the 'normal' C use. C freaks will know the methods, but in scripts they are fortunately very, very rarely used, so if you do not understand them, you'll certainly never need them. Anyway I try again:

floatv() stores a float in the memory space of a var.

fixv() converts a float stored in the memory space of a var to a var. The fixv macro is needed to tell the compiler that the var is in fact a float, not a var.

That explanation is probably not much better, but the only example I know that uses floatv and fixv is the water.c script.