ask about the moving mesh?

aiya

Dear :

In the finite volume on moving meshes, there is the 'Ub'in the equation, Ub is the gird moving velocity.

But in the peric's book 'Computational Methods for Fluid Dynamcis (third edition)',the page of 379, there are these sentences:

'In some implicit time integration methods (so-called full-implicit methods)', in which fluxes and source terms are computed only at the newest time level, grid motion can be ignored everywhere except near boundaries. Examples of such methods are the implicit Euler scheme and the three-time-level scheme, see Chap.6. Since the fluxes are computed at time level Tn+1, we do not need to know where the grid was(or what shape the CVs had) at the previous time level Tn.'

I can not understand these sentences entirely, who can talk about some? Thanks much!

ai. Chinese