When the hamstrings are tight (especially if you insist on keeping the legs straight) they will prevent the pelvis from tilting forward, which means that the bend will come from the lumbar/thoracic spine. In the neutral position your lumbar spine is concave (curves inward), in forward bend you are placing it in a convex position (bulges outward), which means that the discs will be squeezed at the front, creating “anterior disc compression”. Already this is risky for your lumbar spine. That’s why if your hamstrings are tight we usually recommend that you keep your knees slightly bent to allow the pelvis to tilt further forward.