Hi,
I am not sure what the following sentence says;
"Just as spoken languge has sequences of sounds that are not permitted in the language, so sign languages have forbidden combinations of features."
Just as the sound sequence 'sjqza' is not permitted in English, the signing sequence 'clapping your hands then standing on your head' is not permitted in Australian Sign Language used by deaf people.
In any language, there are constraints on what is permitted by the structure of the language.