http://www.vox.com/the-big-idea/2016/10/6/13184492/flip-ticket-trump-pence-constitution

I am not saying this is gospel 0 but Vox is a pretty reputable site

snip.....

The first step of The Flip must begin very soon — ideally, this week or next. At a joint press conference, Pence and Trump should with fanfare announce that, if they win in November, Pence will be in charge, thanks to the rules laid down by the Constitution’s 25th Amendment, which was ratified in 1967 after the assassination of John F. Kennedy.


At the press conference, Trump would solemnly pledge that on Inauguration Day, January 20, he will, only minutes after taking his oath of office, step aside by invoking the presidential disability provisions of Section Three of the Amendment — much as he might step aside if he were undergoing a scheduled coronary bypass that day. He will give notice on Inauguration Day to the president pro tempore of the Senate and the speaker of the house that he is "unable to discharge the powers and duties of his office."