Help, how did I get this backlight effect?

[denzjos]

That's how I did it : select the blue and grey with the 'Fuzzy Select Tool'. Then 'Select / To Path' and 'Select / None'. Make a new black layer, a transparent layer and a copy layer from the original picture. On the transparent layer stroke the path with white Paintbrush (Hardness 12, Force 14, Size 170). Make the original layer copy black : Colours / Desaturate / Mono Mixer (green slider to the left