The poppet vale opens at high water flow and pressure. At low speed and low pressure, the cooling system is designed to get enough water through the jackets to keeps things cool. When speed increases and of course water flow and pressure, there is actually too much water flow. So in order to not put strain on the plumbing, the poppet valve opens and diverts some of the water to the exhaust instead of the normal path through the heads. If the poppet valve sticks open a common symptom is overheating at idle but normal operation at speed.
There are two designs of poppets, the newer one 'fixes' some previous issues. It is important to note the order and orientation of parts when you disassemble the valve to make sure it gets re-installed in the correct order. Yamaha recommends new gaskets also. Personally, if I am not having symptoms of an open poppet and I do not have excessive salt buildup, I don't routinely go looking for problems that probably aren't there ( like the poppet valve). I do the same for the water pump. If all seems OK, only guilt after several years makes to change it.
