This is a real treat. At a live event in France last month, Kenji Ohba, original actor for Space Sheriff Gavan, appeared as a guest. During part of the discussion, he demonstrated for the audience (in slow motion, no less!) exactly how Space Sheriff Gavan poses for his JOUCHAKU! transformation. He even makes sure to show the part where the light particles are flowing down!
Ohba is a pretty cool guy. His history goes all the way back to Himitsu Sentai Goranger. Before he made his first unmasked on-screen appearance as Denji Blue, he was the suit actor for Akaranger, the original first red hero in Sentai history. His first credited role is a monster suit actor in Kamen Rider. Even at 58, he's still a prominent stuntman. He appeared doing stunts in the recent Space Sheriff Gavan movie where he once again transformed and fought alongside his successor -- and performed his own stunts in several unsuited combat scenes.