Lipo cells are good until they drop below 3.0 volts. If you have a 2 cell lipo (3.0 x 2cells=6.0volts) So if a 2 cell lipo drops below 6.0 volts it will ruin it.
the receiver works down until 3.5 volts so if you running 2 cell it would wreck the pack to keep running it.

You don't have to worry about this with newer escs (ex. Tekin Fx pro) that have built in voltage cutoff you can adjust.

Hope this helps

Ryan.