Switching activity file also known as SAIF in short, is a file that captures the toggling activity of all the circuit nodes in the design. It is similar to the VCD, except that this is normally much smaller in size and hence easy to generate (takes less time) and manage (less disk space). It can be written out during simulation using VCS/NCSIM etc. and is used by Power compiler for dynamic power analysis. You can read in the SAIF file in place of a VCD file.