I have a question regarding the frenulum breve as I don't quite understand exactly its effects.

Does the frenulum breve restrict the complete erection of the penis, restricting the expansion at its full length or does it only restrict the full retraction of the foreskin over the completely erected penis ?

Sounds as if you've been reading at some pro-circ sites which encourage circumcision to free the penis from the restritions of the foreskin which prevent it from growing to its full potential. Is that right?
Well, your penis when erect is as big as it's going to get, regardless of what you do to it. The frenulum is stretched when tension is applied by moving the foreskin. The tension comes from the lower part which is moving, and in turn, that causes the pull on the upper part which is attached to the shaft, thus causing a downward pull on the end of the glans.