The combo itself is simply as follows: single jump f-air, midair jump d-air. However, some players refer to the entire chain of n-airs and f-airs followed by the spike as a "Ken combo" due to its effectiveness in SSBM competitive play. However, it should be noted that players can usually escape this combo by DIing the f-air out.
In Brawl, the Ken Combo can be executed by other characters such as Toon Link, R.O.B., and Olimar, although it should be noted that it is not a true 'Combo'; lack of hitstun allows the victim to simply airdodge away or even hit back. Additional hitstun however in different versions of Brawl+ allow for even more characters to perform it, and not enough time for an opponent to airdodge away.