In paschimottanasana you are stretching all the way from the heel, the back of the leg and up through the spine, and the top of the head. Sitting tall with your feet extended in front of you, keep your feet flex. By engaging the quadricep muscles, that helps to bring up the kneecaps and that also starts to relax the hamstring. Anatomically speaking, muscles work in pairs - one contracts and the opposite relaxes and stretches. It is important to remember this because we often think of yoga as stretching but really we are also doing the work of contracting muscles. Yoga is a very effective body-weight exercise.
Imagine that there's space between the vertebrae and you are lengthening through the spine. Inhale, lift your arms up. Exhale, fold forward from the hips keeping your spine straight as much as you can. Stay here for five long breaths.
Each time you inhale, lengthen the spine a little bit more and each time you exhale, go further down. Do micro-movements each time- inhale, lengthen the spine, each time we exhale, go further. After five to ten long breaths, keep reaching to the front of the room and rise up. Exhale and relax.
The two most common injuries that you'll encounter in yoga would be knee issues and then lower back issues. This is one of those poses where you have to watch out for lower back issues. This is especially contraindicated for students who have a flat-back spine where there is a lack of the curvature of the lumbar spine. The pose puts too much pressure on the lumbar discs and could cause damage.
Some students are simply not bending from the hip and instead bend from the waist. When they do that, this creates a lot of pressure on the lower back. So a few things that you can do to work around this and give the proper modification of the yoga pose. First of all, lengthen through the spine, reach forward, and stop before the spine starts to round, instead of going all the way forward. Another thing you can do to relieve the pressure on the lower back and to be able to lean or bend from the hip would be to bend the knees. The same applies for a standing forward bend or forward fold. The third option is to sit on the block, this frees up your hips and might enable you to fold forward more easily without having to round the lumbar spine.