Many of the bow makers out there offer this as an option but don't know what makes it so different from a standard locator grip. Palm swell is often mentioned as well. Does this mean on the side of the grip where the palm naturally locates, or is at at the backside of the grip closest to the shooter?

You got it,
Grip is built out a bit to fill your palm.Also, the Asbell grip locates your thumb in a lower position.Instead of the thumb touching the index finger.It touches your ring finger.

It is what we used to call a medium-high wrist. It forces the hand/web into the pivot and reduces torque.  I'm not a fan of the bigger palm swell even though I shoot a loose hand.  Different strokes 8^).

So for those that like the grip, does the palm swell contribute in some way for holding ability?

No.  It merely positions the hand to reduce torque and open the grip.  A solid bow arm is what contributes to holding ability.

I should add here that the Asbell grip (medium high) is not for everyone.  You need to try that style and see if you like it.  I like a lower grip as it is more comfortable for me and much easier to tote the bow in the woods all day.  Everyone is different though, and you must try before you buy.

I've been shooting the Asbell grip since 1986 and like it a lot.  When I let other folks shoot my bow they always comment on how much they like the grip.  It just seams to fill you hand and allow for an even spread of the bow weight accross you hand.  It also make you grip the bow the same each time.  Some folks say that it incourages a natural canting of the bow.  
The Blackwidow web page did have a side by side view of the their grip and the AB grip.  It is a fuller grip, so if you like small gips it not something that I think you will like but it's best to try one.  BW does have a bow loner program that you could get a bow from them with the grip just to try out if you really wanted to shoot one.
One thing to remember not all Bighorn bows had the Asbell grip as they would build you other grip stiles if you wanted them and they did offer in the early years a high, med, and low grip as well as the Asbell grip.

DJ

Thanks all. So far I haven't run into anyone with a grip like this on their bow. The "try a bow" gets pretty costly coming across the border so hopefully some fellow 3D'r will cross my path.

I disagree with George - it adds torque, not l or R, but up and down.  The higher the the palm swell the more up and down torque.  In contrast a Hill type riser would have the least up and down torque, because the palm of you hand is parallel to the target.  

Bowmania