One of the symptoms of Imposter Syndrome is anxiety around the performance of tasks. For someone living with Imposter Syndrome, the period between the performance of a given task (eg: presenting an expert paper to a peer group) can be one of escalating anxiety.
Until that task is performed.
As soon as the task is performed, the anxiety immediately drops. When they realised that they have always been able to perform this task all along. Confidence returns at that point.