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The proximal optimization technique (POT) in coronary bifurcation stenting improves apposition and side-branch obstruction. The POT balloon should be positioned with the distal radio-opaque marker at the carina cut plane. However, the real impact of positioning remains unknown. Synergy ™ stents (Boston Scientific, USA) were implanted on left-main fractal bench models. Initial POT was performed in 3 positions according to distal shoulder position (loss of balloon parallelism) relative to the carina cut plane (n = 5/group): i) "proximal", 1 mm before carina; ii) "medium", just at carina; iii) "distal", 1 mm after carina. Results were quantified on 2D- and 3D-OCT. Compared to implantation, initial POT improved malapposition in all positions ("proximal": 61.5 ± 1.4% vs. 5.1 ± 2.7%; "medium": 60.2 ± 2.4% vs. 1.3 ± 0.6%; "distal": 60.5 ± 2.9% vs. 1.1 ± 1.8%, p < 0.05). However, residual malapposition was greater in "proximal" position (p < 0.05). "Proximal", unlike "medium" or "distal" POT, also failed to improve side-branch obstruction. Conversely, "distal" POT significantly overstretched the main-branch ostium, with stent/artery ratio 1.22 ± 0.04 vs. 1.11 ± 0.07 for "medium" POT (p < 0.05). Shoulder positioning is essential to optimize the mechanical benefit of POT without main-branch overstretch (too distal position). Experimentally, the best position is just at the carina cut plane ("medium"). • POT balloon positioning is crucial in optimizing mechanical benefit in terms of apposition and side-branch obstruction. • The POT balloon should be positioned so that distal loss of parallelism occurs just at the carina cut plane. • Achieving this requires perfect knowledge of the specific balloon characteristics. [ABSTRACT FROM AUTHOR]