Stabs are used to stiffen the BHA, preventing wobbling down hole. The more full the gauge the more the string will not move, or bend. Full gauge tools are not used in directional work for this reason. The underaged stabs are used to minimize wobble in the string , while still offering flexibility to make a curve.
A Straight Blade Stab is utilized for sliding, when the drill string is not turning, only the bit at the end of the motor has rotation.
A spirial bladed stab is designed to stiffen the BHA while the string is rotating.