what he did is wrong .he is putting 8.4 volts into a 4.2 volt circuit...smoke is bound to happen..and your pic is for a bullet connection nothing close to His setup..

a 2 cell lipo is wired in series 2 cells @ 4.20 volts max each off those they have a universal jumper for Negative of 1 lipo cell to Positive of other lipo cell #2.. he has a Negative going to balance plug with 4.20 volts and he also has Total volts going to 1st cell balance lead from charger.. That's it wrong smoke blown charger bad will happen..

Must be setup like this
+ 2nd cell 4.20 volts
-+ shared
- 1st cell 4.20

He has it..
+ 1st and 2nd cell combined 8.40 volts
- wrong needs both cells -+ away from charge current only 4.2 volts acros outer terminals NOT 8.4
- nothing incomplete connection