I recently bought Summit 1/16 (not VXL, hope I can put it here). I think about undergive gearing (found some threads about it here as well), but there is one thing I don't understand. In the user manual there is Gearing Compatibility Chart. It says that when I go for a really small pinion gear I need to replace the stock motor (Titan 12T 550) by aftermarket 540. Do I need to replace it when I go for undergive gearing as well, as in both cases I increase the gear ratio?

What is the reason for replacing it (when replacing pinion gear) anyway? I think I understand the need for second battery when decreasing gear ratio (engine stressed more, draining more power from batteries) but when I increase it?