User blog comment:B214/Question and Answers 2/@comment-4547267-20140604230500/@comment-1359322-20140605001015

That is Sword Birth's ability, to create swords. The Boosted Gear doesn't have that ability. Yes the Boosted Gear would remain with Issei even if his left hand is severed, but unless if Issei reattaches the left hand, he can't summon the Boosted Gear.