The defence of impossibility would be raised where, as the word suggests, it is not possible for the person charged, to have committed the crime alleged against them.
For example, if a person was in Sydney, at the same time that a car accident occurred in Melbourne, and they were subsequently charged with that offence, it would be an impossibility. It is important to note, that in cases where an alibi is to be raised, the defence are required to advise the prosecution in advance, of where they assert they were, and who they were with, so that they can have an opportunity to conduct their own enquiries, and cross-check the information provided.
Most of the time the defence of impossibility is just run as the Prosecution being unable to prove one of the elements of their case
If you have been charged with an offence, and you think this defence might be open to you, you should call a lawyer to discuss your options in more detail.