The suicide handle is physically, the same thing as the Oh Shit handle, with one important psychological difference...
Its use is reserved for the handles in the cars of those with whom riding in their car can be a possible life-ending experience.
OS bars are for mildly gripping during the infrequent manouever where the lateral G-force acting on your body is around .3Gs. The suicide handle on the other hand, is used to hang on for dear life during the frequent, greater than .6G manoeuvers that the driver negotiates.
On a related note: In my experience, the people whose cars don't have suicide handles, are generally the ones who need them. Case in point, myself. A few of my friends are too scared to ride in the car with me, after a few spins, and gravity shifts that gave the feeling of two-wheel motion. :)