Hamstring tightness causes the pelvis to rotate backwards and reduces mobility in the lower back. This makes it difficult to find the neutral spine position in the address position.
Massaging the hamstrings will help improve this. Keep breathing throughout the process to help get oxygen to the muscle and only ever put pressure through the roller when massaging up towards the heart.
Check out more from Jon Bowskill, TG's fitness guru, at: www.jonbowskill.com.