I am aware that during nuclear fusion for light elements and nuclear fission for heavy elements, the resultant elements have less mass than the original reacting elements (ie the mass defect) because of the differences in their binding energies per nucleon. This is what allows them to release energy.
Now, from my understanding from reading online, this mass defect comes into existence because of the strong nuclear force — some of the mass of the nucleons is used up to form the strong nuclear force to keep the nucleus intact.
So, if the mass defect is essentially equal to the binding energy of the nucleus, and this energy is equal to the strong nuclear force holding the nucleus together, why is this energy released as usable energy after the fission/fusion? Isn't this the energy keeping the nucleus intact? How can it be released and still leave the nucleus under the influence of the strong nuclear force?