How to use HRM-env event?

On mine, if the watch is on its back I see it reporting 1 when there's nothing there (eg the green LED ends up being off) and 0 when I get my hand close to the sensor. If I shine a torch at it or point it towards the window, the value rises.

The real use it for something like the flash counter though: https://banglejs.com/apps/?id=flashcount

For that you have to manually configure the sensor to run faster and to turn off the green LED:

Bangle.setOptions({hrmPollInterval:5});
Bangle.setHRMPower(1);
Bangle.setOptions({hrmGreenAdjust:false, hrmWearDetect:false, hrmPushEnv:true});
Bangle.hrmWr(0x10, 197&0xF8 | 4); // just SLOT2
Bangle.hrmWr(0x16, 0); // force env to be used as fast as possible
Bangle.on('HRM-env', function(env) {
    console.log(env);
});

So then it'll run nice and fast (200Hz in that case) and you could use it for some kind of sensing or communication