Dr. Levine describes 'pendulation' in his book, In his book In An Unspoken Voice on p. 79:

"(Pendulation) is the body's natural restorative rhythm of contraction and expansion that tells us that whatever is felt is time-limited...that suffering will not last forever."

He tells us that we can help clients understand that feelings can and will change. If we don't realize this, we feel 'stuck'.  People who experience panic attacks can feel terrified that they were going to be in that state forever. Knowing we can 'go through' the emotion, that it will not last, can be a powerful tool for healing.