When you are bending forward, you must keep the following points in mind.
Hinge at the Hips:
For a healthy forward bend, you must use a forward hinge at the hips with a straight lower back. The length of the back ought to be maintained all through the bend. Such a bend does not affect any disc anywhere in the spine. The discs remain decompressed and are not under any strain.
Wrong bending can be a threat to the back but a healthy bend at the hips is a beneficial exercise for the muscles of the back as they get a healthy stretch. Such a bend brings muscles that run along the length of the spine into action. This keeps the back aligned rather than rounded. As these muscles strengthen, the hamstrings get a healthy stretch and tone.
We are taught to bend our knees to preserve our backs. This does preserve the back as it does not round. However, it stresses the knees and reduces the opportunity for the hamstrings to stretch and for the back muscles to strengthen.
In general, you must avoid bending forward with the knees bent too much as this can put undue pressure on the knee joints. You can bend knees for lifting heavy objects or when the back is injured or in pain.
Bent knees also facilitate a hinge at the hip for those with tight hamstrings. In such cases, tight hamstrings pull the sitting bones (to which they are attached) down. This forces the pelvis to tip backward. Bent knees ease the demand on the tight hamstrings and facilitate the hinge forward at the hips.