In order to offer student teachers training conditions in the field which take into particular account training aspects that make for professionals, the teacher education college in Valais (Switzerland), concerned with closely linking training in the college and training in the field, has set up a training scheme specifically aimed at trainers in the field: training course teachers known as “practitioner trainers” (PF). The log books play a key role in the different methods put forward for helping the expert support of students in training. This contribution highlights the role of potential space – open, comfortable and stimulating at the same time – attributed to the practitioner trainers’ training log book, with the aim of facilitating access to writing connected to oneself, as a person in a private and professional capacity. It also takes up the concept of a steward who, through suggestions of writing activities, guides the one being trained along the path moving from the use of writing as containing information to the task of distancing and dealing with this information. This task has a knock-on effect which, in fact, contributes towards self-training of the subject.