This is a progression from the straight pole vaulting into creating a bend in the pole and keeping the pole moving. We need to keep the pole 'rolling' after take off and keep the movement right the way through to the vertical.
The vaulter approaches and aims to keep the pole away from them as they jump and hold the take off position right through as the pole goes to vertical and beyond. Getting the right pole for your body weight and strength is important. If it is too light bend will come easy but you risk over bending as your technique improves. If the pole is too stiff then a huge amount of left arm strength might be needed which in turn can lead to injuries.