The primary intent is to help you refine intonation or tonal quality through drilling and listening to yourself.
You can either play a passage of music with your instrument into the practice echo or just sit there and do "I know you are, but what am I?" until you get tired of it.
The main feature is a "quiet detector" that triggers the start and stop of the recording via the microphone input.
Try it (in Chrome or FFOX) at https://practice-echo.herokuapp.com
Thanks to https://github.com/cwilso/volume-meter.git for the volume meter example.