The characteristic of this kind of nozzle is that part of the way along the inside of the nozzle there is a straightening of the curve of the nozzle contour, followed by a sharp step outwards.
At low altitude, this causes the jet to separate at the step and ambient pressure maintains the jet at this place, avoiding jet instabilities and avoiding massive overexpansion.
As the altitude rises, the jet becomes progressively under-expanded and grows until it fills the nozzle, at which point the gas provides more pressure against the rest of the nozzle and thrust and specific impulse increases.
^Génin, Chloé; Schneider, Dirk; Stark, Ralf (2021), "Dual-Bell Nozzle Design", in Adams, Nikolaus A.; Schröder, Wolfgang; Radespiel, Rolf; Haidn, Oskar J. (eds.), Future Space-Transport-System Components under High Thermal and Mechanical Loads: Results from the DFG Collaborative Research Center TRR40, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, vol. 146, Cham, Switzerland: Springer International Publishing, pp. 395–406, doi:10.1007/978-3-030-53847-7_25, ISBN978-3-030-53847-7
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