By taking Falco's laser and immediately spamming Turnaround up-tilt, any of his aerial approaches will trade or be obliterated, and the trade is very advantageous for you. Leffen was probably the most prolific up-tilt spammer amongst the top Foxes.
Falco does have counterplay, of course, but even just forcing him to think twice about laser-aerial approaches complicates his game plan.
“ the "leffen utilt" is really obnoxious to deal with for falco mainly because of how favorable the trades with approaching aerial are. just hold turnaround utilt angle, take laser, press a, win game. make sure to tweet about how much harder fox is than falco afterwards ”
— cookbook.gg