Company changing to self-insured health insurance questions?

The company I work for was bought out, the new company offers a SELF-insured health plan that the employer pays out the claims. I know that it is through Aetna as the administator of the plan and it is a POS plan. Can anyone tell me the way a self-insured plan works and what are the drawbacks? I am worried that I am not going to have the coverage I did with my current HMO plan.