everyone from my area doesnt get hit by it anymore because they learn, but thanks for the unfounded insult Cort =)
Like many others have said, it's about the mix up.
c/p'd from my other topic, but I had another one before that was more in depth:
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Slap is best used for pseudo resets and getting your opponent off balance. You can FC nair to slap, and the otherwise techable nair becomes a no knockback nair-slap from which you can keep pressure going with other aerials or a grab. Slap does not technically straight combo into anything, it relies completely on the opponents lack of reaction (which is surprisingly reliable). Most people don't react or don't realize they can move almost immediately after the slap, and your next attack is coming fast (or it should). The thing is that they have to realize they have been slapped as they are being slapped, or else its too late.
Double slap doesn't combo into anything either, again following anything after a double slap is just a pressure game waiting for your opponent to error. It can easily backfire, and the main thing to keep in mind is to mix it up. Peach doesn't have too too many actual combos, but she can mix up her game so much that you can pile on pressure and force your opponent into awkward situations. Say they expect you to attack after a double slap and decide to side step. A favorite mixup of mine is to double slap, dash dance away and back again into a dash attack. It ruins their timing and allows you to do some actual combos or damage. You can also simply run up and grab, or jump forward and dair, or any one of other options. Double slap is good because it starts a game of rock paper scissors that is in your favor. Usually. I wont make promises about how that works against marth or fox
FC nair double slap is something that seems to be popular now and I kinda blame myself for it being like that. Nair double slap is good, but once again only in the way it is one of many options to keep pressure going. Alot of people try to do something out of their sheild prematurely, or mess the timing up after the second slap allowing you to continue with something else, but there are ways to get around nair double slap if it's something you know is actually coming. Some characters can actually grab Peach between the two slaps. It's actually a little absurd . Most anyone can grab after the second slap, and anyone can simply roll away.
There you are, rolling away solves everything.
But what if I decide to only do one slap and then d smash? Now their attempted sheild grab got turbo eff'd. Or I can do a solitary nair and follow up with a mid float dair moving forward, either pressuring their still sheild or getting them out of their roll. I could do nair slap grab, or nair dash dance dash attack, or one of many options.
To restate it, nair double slap is only as good as it is unexpected.
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But yes, you can chain nairs against floatier characters, peach is one, Mario Bros are another, bowser and mewtwo and according to Dogy Yoshi as well, but I don't play against/care enough about yoshi to know that.