Ideally your low back is resting on the bench along with the hips and shoulders. It may help to use a rolled up towel to slightly lift the head to achieve this. As you press the dumbbells up and down, the back remains relaxed into the bench -use your low abs to do this. As you tire, you will find the back wants to start arching. If you can’t stop this arching after 2 reps, it is best to stop the exercise since your form is breaking down and you run the risk of injury.
The elbows may not be completely close to the sides, but they will feel magnetised to the sides instead of flaring out willy-nilly. This helps to engage the shoulder stabilisers properly.
At the bottom point, aim to feel a stretch across the chest whilst being pinned to the bench.
As you press upwards, the dumbbells should be following a natural curve upwards and backwards – towards the head. This is a good sign that the shoulders are stabilising properly.