The specific example I saw recently was cleansing of concrete (portland cement) film from brick using a diluted muriatic acid. It appeared that the more scrubbing was used, the quicker the cleaning process would go. That seems to match up with everyday experience.

However what is not clear is how that mechanical agitation was specifically improving the reaction rate. 

More generally, are there a 'class' of reactions or reagents where mechanical agitation will alter the reaction rate? And is that affecting the actual reaction or more just the process efficiency?