Cwatters- The Harrier seat has two pitots which flick out into the airflow post mdc detonation . They sense the forward airspeed and decide on a 'fast' or 'slow' ejection depending on what they find. Obviously speed is far more critical in the case of a Harrier ejection in terms of what the seat has to do compared to most fast jets having almost exclusively reasonable forward airspeed .
Modern seats are far more gentle on the occupants due to the rocket pack . However posture is still a major factor in reducing spinal injuries.