I'm in doubt about how I am supposed to rev the engine without engaging the gearshift on my Bayliner 1750 Capri with OMC 2.3L engine from 1988. The throttle handle, which is a US Marine, has an nonworking button underneath the horisontal part of handle. Should that be the button to disengage gearshift so I can rev the engine without moving? I hope someone can clarify it for me. Thanks.