Benefit: To use this feat, you must make a single unarmed attack as a standard action. If this unarmed attack hits, you deal damage normally, and the target's base land speed is reduced to 5 feet for a number of rounds equal to your Wisdom modifier unless it makes a Fortitude saving throw (DC 10 + 1/2 your character level + your Wis modifier).
Concerning the bolded part: I'm not a native english speaker an thus I am unsure of how to interpret this.
Is this wording equivalent to that of the cleave feat, which lets you make a special standard action attack (not the attack action) as part of the feat or can you use this feat whenever you "make a single unarmed attack as a standard action", meaning that you add it as a rider effect on top of any standard action unarmed attack that you make?
TL;DR: Is making a standard action unarmed attack a prerequisite for or a consequence of using the feat?
By Paizo's standards in wording it means that scorpion style is a special standard action like that of cleave.
I agree with avr. A conditional ability would be phrased as a conditional sentence, i.e. start with "when" or "whenever"; compare the Heriot Knight's Mighty Strike ability.