The Incidence of a Tax

In this tutorial you can choose the excise, or tax per unit, to be levied. Initially the diagram shows a zero tax. Choose a positive number for the tax. The net price (netP) is that received by the seller. The gross price (GrossP) is that price paid by the buyer. The burden of the tax is shared by the buyer and the seller.
You can change the burden of the tax by changing the slopes of the demand and supply curves. What does the incidence of the tax have to do with the elasticities of supply and demand?