Wow, this long-dead thread got all tangled up in its funeral shroud when it was resurrected.
To clarify:
In NFHS rules, the plate umpire is not required to get help on a checked swing when the defensive team requests it. As a matter of prudence and game control, I always do request help when asked, but high school rules don't require it. (They also don't require me to wear a cup, but it would be stupid not to.)
On any play where an umpire seeks help from his partner, the call remains his own, and the call doesn't get changed until he accepts his partner's input. Even when the field umpire says, "Yes, he did!" it's not a strike until the plate umpire raises his right hand and calls it a strike.
This principle is true of all plays on which umpires regularly get help, such as a pulled foot or swipe tag at first. Help on a checked-swing call differs only in that the consultation between umpires occurs in public, but the principle is the same. The umpire responsible for the call asks for more information, considers it in light of what he saw, and either changes or does not change his call.
So what if the plate umpire is sure the batter held up and his partner thinks otherwise? I could stick to my guns, incite a riot and discredit my partner, but I would never do that. Even if my partner is a complete doofus wearing a clown suit and signaling outs with a bicycle horn, he is still my partner. If I publicly ask him for help and he publicly says it's a strike, I will ring up the strike. (Besides, strikes are good and he just gave me license to call another one.)