Being the first on the scene of a waving yellow, sitting up and raising an arm to warn following riders is the 'respected' way of acting. Clearly, slowing down is the correct thing to do -- but exactly how much really depends what a rider actually sees. The no passing rule, for any yellow -- stationary or waved is of course vitally important in avoiding any further attrition, and it is nice when leading riders do raise an arm. I've more than once been made aware of yellow flag incidents, around blind corners -- by thinking riders that have the common sense to warn following riders. Yes - the Yellow flag rules are clearly very important, but more so is the unspoken etiquette used by most thinking riders.