By locking the hubs, you are spinning the entire front drive train - so when you engage 4WD, you're engaging a spinning gear with another spinning gear (both of which are going the same speed). With the hubs locked, it's shift on the fly (into high range).
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner