BLE is any Bluetooth Low Energy device. But when you start it from the watch, the device must have been saved on the app first since you can't scan for devices from the watch. To do this, go to phone app > Start Activity > Scan for BLE devices. Wait for the device to show up in availbale devices and tap to move it to saved devices. This only needs to be done once.
After this, when you start activity with BLE on watch, it will automatically connect to the saved devices, if available.
Unfortunately, my Pebble2 did not advertise itself correctly as an HRM device(I checked it thoroughly) so I could not test it, but I hope that is a fault in my watch and not all. But I have tested with other devices and prepared it to work with Pebble too. Do let me know if you can get yours to connect.
In background, you can allow Accurate Tracking from the phone app to get GPS data(not BLE).
Those summary notifications are shown for somewhat longer activities and updated on the phone (and Android Wear watches) silently whenever you pause. Unfortunately on Pebble watches I cannot make them silent, so you will get each update with a vibration.
The HR chart you probably saw might be for the whole day. That shows your HR trend for the day.
Activities will show a small arrow on the right when more data is available. Clicking on this will show maps and graphs depending on the available data(you might have seen it for your sleep data). But it shows HR only when higher frequency HR is available. Use Pebble as BLE HRM to get this.
Thanks for trying it. Keep me posted if you have any questions or feedback.