Imagine walking and not thinking about your stride--just doing what feels natural. It's not hard because generally we don't think about it -- we do what's natural depending on the pace we want. Generally, the higher the pace we want the shorter and quicker the stride will be. Given the arbitrariness of a fixed crank length, it's the "shorter" aspect of turnover that is missing but in cycling, instead of 'stride' it's foot speed that you cannot make 'shorter' with a fixed crank length. So, if you wish to have a higher pace you know you'll need more turnover (gearing aside for the moment) and the only way to 'shorten the stride' will be to preselect a shorter crank length so as to lessen foot speed --i.e., so you can do what feels natural. And, while it may be different for different folks, the optimum may be higher cadence with lower foot speed -- achieved with shorter cranks -- while upping the inches per revolution by shifting to a higher gear.