probably nmb click tactile switches.
i could never understand the reasoning for having a very large backspace key right at your thumbtip. if you make so many typing mistakes that you save a significant amount of time with erase eaze versus having the backspace key in its normal position, you should probably work on your typing accuracy. besides which, the split is right where my thumb normally hits the spacebar.
on the alps glidepoint keyboard, the erase eaze behavior is configurable - hitting ctrl-alt-shift-erase eaze switches between left-side erase eaze, right-side erase eaze, no erase eaze, and both erase eaze.