In a neutral alignment the force of your skeleton and gravity pass through the middle of the pelvis and through the bones of the leg.
In a posterior tilt the weight of upper body sinks into the lower back or the sacral region, and forward toward the toes. When you attempt to bend or squat with a posterior tilt the back is forced to bend somewhere (typically the lower back) causing strain in the spine and knees.
However, sticking your bum back allows for a counterbalance for the thing you are lifting. Loading gets transferred through the legs and down to the heels.
Fortunately, we can relearn how to stand and bend so that our pelvis is hanging out in the best place for counterbalance and support!
One of the key things to sense for are over contracted abdominal muscles. To tip your pelvis for lifting (as seen in the picture above, the abdomen needs to lengthen so that the lumbar muscles can shorten creating a neutral spine. Squating to lift or reach something from th ground should feel easy and stress free on your back and knees. Ask an experienced Personal Trainer, Pilates Teacher, or Feldenkrais Practitioner for help if you are struggling with keeping your behind behind!