with fancywrap, with -x foobinary fooname foocommand:
foobinary always becomes the interpreter, regardless of foocommand:1
ps shows fooname instead of the interpreter as interpreter
for foocommand, ps |grep $$ show the "real" interpreter

with fancywrap, without -x, only foocommand:
foocommand:1 decides about the interpreter, fails if wrong

without fancywrap, the behaviour _is_ accordingly.

probably the shell uses the same execv() function.