A carbocation undergoes a 1,2-hydride or 1,2-alkyl shift when the shift leads to:

Rearrangement is driven by stability: a $2^{\circ }$ cation converts to a $3^{\circ }$ cation, or a $1^{\circ }$ cation to a $2^{\circ }$ or $3^{\circ }$ one. A shift to a less stable cation does not occur as the main pathway.