Amish life is written about a great deal and is usually written about wrongly, as a set of prohibitions with a period costume. The prohibitions are real but they are downstream of something.
The governing idea is Gelassenheit, which is usually rendered as yielding or submission and means the surrender of self-will: to God, and through the community as the place where God's will is worked out. Pride, self-assertion and standing out are the faults it opposes. Plain dress is not a uniform preserved out of nostalgia; it is the refusal to distinguish oneself by appearance. The same logic runs through the reluctance to be photographed, the absence of decoration in the meetinghouse, which is simply somebody's house or barn, and the choice of ministers by lot rather than by election, which removes ambition from the process entirely.
Technology is decided the same way, and this is the part most often misdescribed. There is no Amish doctrine against machines. Each church district's Ordnung is reviewed twice yearly, and the question asked of any technology is what it would do to the community: whether it would pull work out of the home, put distance between households, create differences of wealth, or bring the outside world in without a filter. That is why the answers vary so much between districts and why they are not consistent by any external logic, a telephone in a shed at the end of a lane, a diesel engine but not a tractor in the field, a hired driver but not an owned car.
Baptism is taken as an adult, usually between sixteen and twenty-three, and everything else rests on it. Rumspringa, which the outside world has made into a spectacle, is the period before that decision in which some latitude is allowed, precisely because the commitment has to be freely made by someone who has seen the alternative. The great majority make it and stay.
Because baptism is a free adult vow, breaking it is treated as a broken vow rather than as a difference of opinion, and that is the reasoning behind shunning. It is also the point at which outside observers and Amish communities most often fail to understand one another.