One of the most terrifying experiences in my life was teaching my 15 yo firstborn to drive. (I taught my other two children too, but the first one was by far the most terrifying one!)
It was the first time in my parenting experience where I had to hand the controls to my child and with it, the responsibility for 2 or more lives. It was the combination of the lack of control and the inherent high-stakes risk that terrified me. By the time he got his license and I was ready to trust him to drive by himself, I've made dents in the passenger seat armrest from clenching it, and on the floor - from hitting the non-existent brake pedal.
Unfortunately, driving is one of these skills that can be learned only by practice - no book, training class or PowerPoint presentation can fully prepare a driver (nor the teacher) for the experience. Learning how to drive, while guided by someone experienced, shortens the learning curve and avoids terrible accidents.
Similar is the case with Agile transformations. A team cannot become Agile only by the virtue of taking a class together. They need first-hand practice and guidance from an experienced Coach to help them learn quickly and avoid the pitfalls.
Some have likened coaching to "driving from the back seat." It is a great expression, but an inaccurate one.
Coaching is like driving form the passenger seat - where you still don't have much control, but your life (i.e. coaching reputation) is at risk, even more so than the driver's life.
"Driving from the back seat" is safe. It is a lot more akin to consulting where you give your best advice, only hoping the client would take it, and if they don't - they end up in the wrong place on the map and it is their fault.
Coaching, on the other hand, is a lot more high-touch and intimate than consulting. It takes a great deal of trust from both parties to engage in and accept coaching. You share not only closer physical proximity with your driver (i.e. team) but also more responsibility for their success or failure. As a Coach, you are supposed to guide them, warn them, assess the situation with them, and help them make the right choices. They are supposed to do the hard work of steering the wheel, pressing the pedals, looking in the mirror, and navigating the surroundings.
As a Coach, you are still 100% present and on your toes (remember that armrest!). You watch for every micro decision they make even before they reach for the pedal or the turn signal. In most cases, feedback and hand-signals are enough. But in that one-in-a-million case when you see them taking a too sharp of a turn and heading toward the cliff - you may need to jump and grab the wheel. (And yes, once and only once, I had to throw out my eldest from the driver's seat and complete the trip myself.)
A good coach, knowing when to intervene and when not do, makes the difference between a bumpy ride with someone who becomes a better driver at the end, and a catastrophe (literal or figurative).