Technically, AMSI is a set of DLLs being asked for a buffer evaluation (saying it's safe/unsafe). It means, processes (such as powershell.exe) load such DLLs when want to use AMSI. And it sounds like perfect opportunity to misuse such DLL as a method of persistence.
Registering with Regsvr32.exe FakeAMSI.dll, unregistering with Regsvr32.exe /u FakeAMSI.dll
DebugView for displaying the effect - user name and process name.