Bending vibrations:

The bending vibrations are said to occur when there is a change in the bond angle among bonds connected to a common atom. In a few cases, the movement of a group of atoms along with respect to the remaining atoms in the molecule also causes bending vibrations for a polyatomic molecule. A linear triatomic molecule has two bending vibrations as shown in Figure, while a nonlinear triatomic molecule has only one bending vibration. The bending modes also are of different types.  These are called scissoring, rocking, wagging and twisting.  For methylene (-CH₂) group, the possible bending modes of vibrations are shown in Figure. The 3N - 6 rules does not apply here because - CH₂   group is not a molecule but only a portion of it.

Figure: Vibrational modes of methylene (  -CH₂) group