From Valve Developer Community
This entity is obsolete; its use is discouraged and it may only exist/function in older engine branches.
concussiveblastis a point entity available in the Half-Life 2 series. This entity appears to have been intended for
weapon_cguard, an unused weapon from the Half-Life 2 Beta. It is now used by striders as the centerpiece of their cannon blast, functioning as the explosion that deals the actual damage and creates the warp effect. This entity serves no use to mappers, therefore it has been marked as obsolete.