A formation of pockets, explained by a passive front model, is controlled by breaking a vibrating air/methane flame at its top. Three types of pockets were observed droplet-shaped, spade-shaped ' and string-shaped. Using a near stoichiometric mixture, droplet-shaped pockets were quantified at 0.355 atrti. From either their hydraulic diameter, or their axisymmetrical volume and area, global quantities such as stretch K, burning velocity Sd and curvature Co are obtained. Sd, at least twice the laminar burning speed SL, reaches a maximum ranging from 3.5 SL, to 6.4 SL. K is correctly expressed by the product - Sd Co; from Poinsot el al. correlation connecting burning velocity to high curvature and strain, global relations written between Sd and K, and Sd and Co on the other hand, concur with data. A minimum experimental diameter at the final stage of combustion is found; its value is consistent with that of minimum diameter given by Poinsot et al.