"Momentum building" refers to the gradual increase in speed, strength, or progress of something. It means that something is gaining momentum, becoming more powerful, or moving forward more quickly. Full definition
There is in fact unprecedented momentum building in support of SEL from educators, administrators, and funders across the country. (novofoundation.org)
Its dual - clutch transmission lags as the throttle is applied and thereafter valuable moments are lost as momentum builds. (motoring.com.au)
The more people who understand both how urgent the crisis and how affordable and practical the solutions are, the more momentum we build for the transition to a clean energy economy. (climaterealityproject.org)