The velocity of the compressed layer as a function of time estimated using momentum balance and by assuming constant deceleration. The points are extracted from p–v diagrams/line profiles from synthetic data cubes of simulations (which we will discuss in Section 3). The width of the secondary peak in the broad bridge or ease with which two distinct velocity components can be identified determines the size of the error bars. The left-hand vertical line denotes the time after which the small cloud is compressed. The right-hand vertical line denotes the time after which the smaller cloud has punched through the larger.