Newbie to the series with some questions on melee combat

For using the chainsaw attack, most of the time I have tried it it doesn't work, is there some special condition you need for it to work, like do enemies need to be stunned or at lower health or something? Do you need to hold down the button or can you just tap it to do it? so far I've only once done the chainsaw on chainsaw thing with an enemy, but I didn't know what to do to not die, there was no instruction on that, so when that happens what do you press to not die? Lastly what's the point of executions? They take so long to do leaving yo totally vulnerable, just a quick burst of gunfire is much faster, right?