Simply put: Enhanced haki.
It is stated by Silvers Rayleigh that haki is limitless, and can be infinitely expanded upon. It is also stated that the metamorphosed and focused capabilities of haki vary from individual, such examples include but are not limited to Monkey D. Luffy and Rayleigh himself; with Luffy possessing the ability to maintain it in his arm fusing it with Gear 3rd tincturing it black while on the other hand Rayleigh can just call forth an invisible barrier.
Haki possibilities and explanations
This would also perfectly explain the aggregation of many abilities down to Fishman Karate and possibly even the skill of swordsmanship as it is stated every individual holds Kenbunshoku and Bushoshoku haki but few actually ackowledge the powers and even fewer properly trained to utilise such abilities.
Note: This was indeed a short blog, but I hope you enjoyed the links to assist you in finding the articles to read about if your overal knowledge was lacking. I also recognise I did not go overly detailed with this, but just enough to capture the general idea.