The Matza Dough Must not be Left Idle for Even One Moment
In Siman 459:2, the Mechaber, Rabbi Yosef Caro rules that extreme caution must be exercised to ensure that the dough is continuously worked. This is emphasised by his seemingly unnecessary repetition, after having said, "the dough must not be left idle" - with the redundant, "not even for one moment".
The Mishnah Berura explains this to be a stringency of greater magnitude than what we may have otherwise assumed. It is well established that the dough is extremely susceptible to becoming Chamets when it becomes warm. Accordingly, we may have assumed that the warning of the Mechaber applies only after it has been extensively handled when it may have become slightly warmed.
The Mishnah Berura therefore explains that the dough, from the moment it is formed, must not be left idle for even one moment.
Furthermore, the Mishnah Berura explains that working the dough means that the dough is not just manipulated but that it be vigorously worked. He is explicitly critical of those who do anything less.
He also makes the point that this is not a measure to minimise the risk of inadvertently leaving the dough idle for a protracted period. This is necessary because there is indeed a risk that in even one moment the dough may become Chamets.