User blog comment:B214/Volume 17 spoilers./@comment-24.2.168.254-20140220012350/@comment-1359322-20140220084326

You're misunderstanding something, the incomplete one is the Juggernaut Drive not the Longinus Smasher, the Longinus Smasher is just an attack where is gathers the energy from the Boost to attack.

"[Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost! Boost!!!!!!!!!!]

[Longinus Smasher!!!!!!!]

It echoes through the shrine numerous of times. The sound coming from Sekiryuutei's Sacred Gear."

During the Juggernaut, Issei was in a berserker state, this time he isn't that's the only difference.