Re:How to use a ESU 51900 decoder tester
I don't own one of these, but they look like a useful test kit for checking that a decoder is functional. Looking at the instructions:
You have a 2-wire terminal block (or a pair of "tubes" to push wires into) to connect your DCC command station's track output.
You have a selection of different interfaces to plug in a decoder which has one of those interfaces.
You have a terminal block (possibly push fit for each wire?) to test a decoder that only has wires and not an interface plug. Going by the picture in the instructions it looks like there is labelling behind this block to tell you which wire is which (you'll need to refer to NMRA standard wire colours to match up with your decoder, or LGB wire colour code table for an LGB decoder).
You have a nice flywheel-fitted motor to prove that motor control works on the decoder you're testing. There are a bunch of LEDs that will indicate lights and other decoder functions. There's even a speaker to test sound decoders.
So... basically you plug in your decoder, plug in your command station, dial up address 3 (assuming a brand new decoder) and try out the decoder.
Personally I just use an old motor and a multimeter, but this does look like a handy bit of gear if, say, you fit lots of decoders professionally.