Background Conventional cost-effectiveness analysis assumes societal indifference to the distribution of health gains across potential patients, yet evidence suggests public preferences vary between concentrating and dispersing benefits, depending on the size of health gain. The handful of Person Trade-Off (PTO) studies that explored this did not examine the cognitive mechanisms and rationales underlying the PTO tasks. Objectives To qualitatively investigate how individuals conceptualise and justify trade-off decisions within a PTO task designed to elicit distributional preferences regarding healthcare benefits. Methods Following a pilot phase (n=10), online cognitive debrief interviews (n=10) were conducted with UK adults using a think-aloud protocol and concurrent verbal probing. Participants completed an interactive, web-based PTO task that presented choices between concentrating larger health gains among fewer recipients or dispersing smaller gains across a larger population. Data were analysed using codebook thematic analysis. Results Six primary themes emerged across two categories: decision-making processes and decision drivers. Participants displayed varying degrees of moral discomfort, utilizing cognitive mechanisms ranging from strict mathematical logic (QALY maximisation) to emotion-driven heuristics. Key decision drivers included patient age, quality of survival, societal productivity, and strong death aversion. Participants frequently rejected pure QALY maximisation and held strong egalitarian preferences to give every patient a chance to receive treatment. Conclusion Public perspectives on benefit distribution are highly heterogeneous and shaped by complex normative values rather than pure health maximisation. Understanding these qualitative rationales is essential for isolating genuine distributional preferences from cognitive shortcuts, aiding health technology assessment bodies in evaluating equity weights.