"Use position" gives the solver a place in the sky to start searching from. In KStars, the position comes from the way the mount thinks it is pointing right now. If the position is good, it can greatly reduce the solving time, but if the position is bad, it might not solve at all. The search radius tells the solver how far out from the starting position to search. So if you uncheck "use position" it won't use a search radius at all.