i'm assuming you've already faded your colour out as much as you can without bleach (vitamin c treatments, anti-dandruff shampoo, sunshine etc.)
if you haven't then you can do this without damaging your hair. some articles are:
(same process and both fade but the second one has less ingredients)
if you have already done that what you're describing is a bleach bath (AKA soap cap). you mix up your bleach as usual then mix in shampoo. you put in twice as much shampoo as bleach, so it's a 2:1 shampoo to bleach ratio.
you wet your hair, rub it in, wrap your hair in cling-film to keep the suds in and keep some heat in to help it process faster. wait for the colour change like you were bleaching normally, then rinse and condition.
more details here: