Before revision:  This analysis for the first time informs on the nature of the process under which a protein is attached between the substrate and pulling probe .
After revision:  We provide a rule of thumb to define the experiment protocol duration. Finally we provide a methodology to accurately estimate the number of stretched molecules based on the number of observed unfolding events. Using this analysis on experimental data we conclude for the first time that poly-ubiquitin binds at a random position both to the process under which a protein is attached between the substrate and pulling probe .