Test is a utility module for measuring voltage and current in your modular.
TEST IS NOT A SAFE MODULE!
If you don't know what you're doing, you may damage your rack, other modules, yourself, etc. If you accept the risk, read on:
There are two sets of eurorack power headers on the module. A "goes in" pair and a "goes out" pair. To measure a module's current draw, plug a power cable from your eurorack psu to one side, then from the other side to the module you want to measure. If this doesn't make sense and is not obvious DO NOT BUY THIS MODULE. This module is handy if you know what it does and why you need it, and dangerous if you do not.
For each power rail, there are two square pads and long rectangular pad. In between the square pads, there are 3 x 33Ω resistors in parallel (11Ω equivalent). If you use a volt meter and measure between the two square pads, you can read the current drawn on that rail via ohm's law. For example, if your module is drawing 100mA, you will read 1.1V.
The rectangular pads are grounded. If you measure between the square pad and the rectangular pad, you can read the rail voltage. One side will obviously be lower, according to the current draw and ohm's law. If this doesn't make sense and is not obvious DO NOT BUY THIS MODULE.
Finally, at the risk of not selling TEST or coming off as rude: DO NOT BUY THIS MODULE!
Unless, of course, you understand what it does, why you would want it, and release Super Synthesis of all liability. I use it a few times a year, and am glad I have it.