Here is a close up picture of a disc between two vertebrae with the spine in an upright position whilst the second picture shows the effect on the disc when the person bends forwards.
It can be seen that the disc gets compressed on one side causing it to want to move backwards.
It has been estimated that a person bending over and touching their toes with the legs straight causes an approximate pressure of 225 Kg to be exerted at the point where the two discs meet.