transient simulation for 3D structures in CST

In Advanced Topics manual of CST2006, pp. 51 there is a figure which says that with Transient Solver a "transient simulation" is performed. Can anyone explain math behind this? How is this actually performed?
I understand "transient simulation" in Circuit Analysis where one has lumped elements and voltage/current sources. But how does a "transient simulation" is performed for a 3D structure? How to obtain the Reflected Signal in time domain?

BR
rinox