you are charging thru balance port only thats not going to damage any lipo. I have a charger that charges thru balance port only...when you balance charge a cell you are more than likely discharging thru balance port ..the only thing is you are limited my wire gauge thru balance port the just under 10 amps where main leads can take more amps
The only ones I've seen like that are just cheap low output chargers. Since they are such low output, the balance leads/plug contacts are big enough to charge through without needing to use the main plug for the "fast" part of the charge cycle.
I also use it for my transmitter, I put a 2S 2000mah lipo in that. I could use my main chargers, but it's just easier to plug my transmitter into that charger, then use my other chargers for the main truck packs. I have the axial SX24 USB->2S charger as well, but never used it.
The Powerlabs8 would charge via balance port only at either 2A or 3A (can't remember which) but I never understood why you would want to. Only thing I can think of is maybe convenience because if you put it on that setting as soon as you plugged in the balance cable it would start charging without having to push anything.