when you bridge an amplifier in this way, it halves the impedance "seen" by the amplifier... thus if you connect a nominal 6 ohm load to it, the amplifier "sees" a nominal 3 ohm load... which will kill the amplifier, especially given that impedance curves aren't linear and you are likely to have some pretty significant dips below that nominal load...
which is why they tell you not to do it... not just "idle engineer speak"... it's to keep you from turning your amplifier into a molten pile of metal...