Theories I've heard so far
1, they fix Jacks 'space hopper' time travelling device that's embedded in his hand (bouncing back would explain why they can't get back before the Master unleashes his nefarious schemes)
2, They find the Masters TARDIS - his chameleon device still works, it could have been in the work shop all along without Yana realising
3, The Doctors sonic screwdriver moment was him setting a tracking device that meant the TARDIS would return to him after dropping the Master off in the 21stC.