During gaaras death chiyo ressurected him, she stated only she knew the life transfer technique, well do remember kakashi took off his head protector and used his sharingan to know how this jutsu works.
Now considering tsunade is a legendary healer, it is possible to assume kakashi is smart enough to know tsunade would maybe want this excellent jutsu at her command if need be.
My theory is that tsunade is going to get extremely injured during her fight with pain, realise its over, and before she does die, quickly bring kakashi back, that way her life was not a waste, shes using her 100% death sentence to fix someone elses assumed 100% (at least 95%) death sentence.
Now considering tsunade is a legendary healer, it is possible to assume kakashi is smart enough to know tsunade would maybe want this excellent jutsu at her command if need be.
My theory is that tsunade is going to get extremely injured during her fight with pain, realise its over, and before she does die, quickly bring kakashi back, that way her life was not a waste, shes using her 100% death sentence to fix someone elses assumed 100% (at least 95%) death sentence.