Hello robocod,
FFI is a way to increase performance in Lua JIT mode (and only in the mode). See
http://luajit.org/ext_ffi.html for general information.
It's not intended for general indicators/strategies developers and must be used only when there are known performance bottlenecks in Lua code. It must be used with care: it you pass incorrect parameter to Indicore method - a general Lua indicator will be stopped, but in FFI mode entire application may crash. Also incorrect usage of FFI functions may even decrease overall performance. In the next TS release we'll introduce permissions and FFI permission will be disabled by default for custom indicators.
FFI functions were intended for standard indicators primarily but this API was opened for 3rd party developers too. But one more time - it must be used only when it's necessary and with care.
If you are interested in it we can provide some advices.