If you’re super tight in the hamstrings and hips when you forward fold with straight legs you may find you can’t get the release of the lower back, you may even find you get back pain! As an experiment play with bending your legs in your forward fold, try different depths of bends and feel into the back, see where you can feel a release of pressure and tension. Allow the head to be heavy and to act as a weight to draw the spine down, releasing the spaces between the vertebrae as you decompress.
Then gradually start to straighten the legs, but as soon as you feel the back tighten bend once more, playing like this can bring more release through the whole back body and not just the legs, and can help relieve back ache, neck tension and generally just make you feel looser through the upper body. You may also find that the reason you struggle in forward fold isn’t because of tight legs but because your back is tight so this way you’ll improve your depth of fold.