This variation of the rasengan was made when Buji's master was killed before he could teach him the final step of the rasengan. So he instead used his mastery of Kenjustu to coat his blade in a Rasengan based layer chakra. This took him five years to refine the technique. This resulted in the creation of the Rasenken. Like its parent jutsu and its variations the Rasenken does not require any hand seals. However unlike the Rasengan this technique requires the user to constantly add chakra to sustain it.