  The property of inertia has never been fully explained. A model for inertia
(MiHsC or quantised inertia) has been suggested that assumes that 1) inertia is
due to Unruh radiation and 2) this radiation is subject to a Hubble-scale
Casimir effect. This model has no adjustable parameters and predicts the cosmic
acceleration, and galaxy rotation without dark matter, suggesting that Unruh
radiation indeed causes inertia, but the exact mechanism by which it does this
has not been specified. The mechanism suggested here is that when an object
accelerates, for example to the right, a dynamical (Rindler) event horizon
forms to its left, reducing the Unruh radiation on that side by a Rindler-scale
Casimir effect whereas the radiation on the other side is only slightly reduced
by a Hubble-scale Casimir effect. This produces an imbalance in the radiation
pressure on the object, and a net force that always opposes acceleration, like
inertia. A formula for inertia is derived, and an experimental test is
suggested.
