Troj/PatchLs-A is a Trojan which attempts to create an exploitable vulnerability within the active LSASS.EXE/LSASRV.DLL process. It does this by patching the LSASS process in memory (the files on the hard disk are not affected). This process is known as "injection".
The injected code does not appear to perform any malicious function by itself, but may possibly expose a vulnerability by orchestrating a sequence of requests to the LSASS service manager (similar to how the W32/Sasser-A worm worked)
Although the injection Trojan terminates, the injected code remains active within the LSASS process until that process is restarted - for instance by terminating the process or restarting the computer.
The injected instructions sit idle until invoked by a specific sequence of LSASS requests. They do not by themselves perform a malicious function, but could be used to force a buffer overflow or similar exploitable condition.