A spirit level contains a bubble in a liquid. The level is suddenly jerked forward. Relative to the spirit level and liquid, the bubble moves

As the spirit level is jerked forwards the liquid moves backwards relative to the spirit level due to its inertia, hence, the bubble must move forwards relative to the liquid and the sprit level. In order to accelerate the liquid there must be a pressure gradient in the liquid. It is this pressure gradient which supplies the force which moves the bubble forwards.