Pump supplements (nitric oxide boosters, citrulline, etc.) produce "the pump" via vasodilation — swelling of blood vessels and interstitial space around muscle fibers. This is mechanistically distinct from cell swelling, the intracellular accumulation of osmolytes (lactate, inorganic phosphate, creatine, glycogen-bound water) inside the muscle fiber itself, which is the swelling that actually signals hypertrophy.
A sensor complex — SWELL1 (part of the VRAC/LRR C8 channel), plus integrins and ERK MAP kinase — detects intracellular cell swelling and signals downstream to mTORC1, increasing protein translation and decreasing autophagy/breakdown. Vasodilation swells "the wrong compartment": the pipes, not the fiber. Analogy used in the source: living in an apartment above a lively nightlife street doesn't make the apartment itself more fun.
Pump supplements are still worth taking for non-mechanistic reasons: they make training feel and look more jacked, which improves motivation/adherence (a flywheel effect), can sharpen mind-muscle connection for hard-to-feel muscles (e.g., lats) via kinesthetic awareness, and generate social reinforcement in the gym that indirectly drives more effort and volume. Exception: if a pump supplement reduces how much volume or how many reps a lifter can complete, that's a non-starter.
Because pump correlates with the two variables that most reliably drive growth — proximity to failure and volume — it can serve as a rough "canary in the coal mine" feedback signal for whether a session is well-trained, without being a guarantee of growth itself. Within an individual, the muscles that pump hardest also tend to have the best genetic potential for growth (explicitly flagged in the source as correlational, not causal).
Creatine monohydrate and a carbs + insulin + sodium protocol (glycogen repletion, AKT/mTORC1 activation, suppressed proteolysis) directly increase true intracellular cell swelling and growth — unlike most pump supplements, which mainly dilate blood vessels rather than loading myocytes with osmolytes.
Source: RP Strength / Dr. Mike, "Do Pump Supplements Actually Help Muscle Growth? Evidence, Not Hype."