No matter what it says on the switch or how new it is, idle the engine before switching. It is rare to blow the diodes by switching at idle, even with an old fashioned switch. (Rare does not mean never though)
At idle, depending upon bank depletion, my alternar can put out well over 40 amps. Even a 3-5 amp output is enough to fry diodes! Idling the engine will do NOTHING to save your diodes from being fried if your bank was low when you started the engine.
I run a solar panel so my batts are always charged. My brother accidentally switched the battery switch off, before I shut down the motor, thinking he was helping and it fried the diodes even though the alt was only putting out about 3 amps at the time into a 99+% charged bank...
DO NOT use your "start" battery to start your engine use it as an "emergency" battery and you'll be fine if you add an ACR Automatic Charging Relay. Automatic charging relays only combine the banks when they sense voltages over about 13+ volts and they automatically disconnect at about the same..
In the last 15 years or so I have only once started my engine using my emergency bank and that was due to a stuck bilge float switch that killed my house bank..
Both my house and my start/emergency banks are deep cycle batts so that I'm combining batteries of like age type and size when charging. Contrary to popular belief you do not need a "start" type battery to start a sailboat aux engine..
If your batt switch is breaking before making buy a new one!!