The app is meant to connect to your (https://x-io.co.uk) x-io NGIMU sensors by using your smartphone wifi as a mobile hotspot. You will have to configure the NGIMUs manually to connect to your mobile hotspot in client mode, set one and only one of the NGIMUs as master, set the UDP port to 8002 + index of device starting from zero (i.e. first IMU gets port 8002, second 8003 etc), and the target ip to 192.168.43.1 (this is apparently Android hotspot default smartphone ip). The app will log sensor, quaternion, battery etc data on your smartphone in Android/data/timo.jyu.ngimulogger/files folder with a folder for each UDP port, and folders for each session according to the initiation timestamp. Note, the logged files will be permanently deleted if you uninstall the app so make to copy the data across!

Source code available from https://github.com/tjrantal/ngimulogger. Seb Madgwick from x-io technologies helped implement this logger app by explaining the required steps.

USE

Configure your IMUs manually
Flick on your mobile hotspot
Start your NGIMUs and make sure they successfully connect to your mobile (solid white led)
Start the app, grant access to power savings exemption (app remains active when off focus). Note, e.g. Huawei implement proprietary aggressive power savings options that have to be manually overriden per app. On my phone I had to go to settings->battery->app startup->browse to MovePacer and flick the switch to manual control. Otherwise the app gets killed a few minutes into the session.
Complete the activity you wanted to record. The app records as soon as it is on. You can switch away from the app and it will keep running in the background
Restart the app and hit "Terminate App" to finish recording and to shutdown the app
Copy data across from Android/data/timo.jyu.ngimulogger/files for offline analysis