Essence of the Phenomenon: Reaching orbit is not the "disappearance of gravity," but rather transitioning into a state of continuous freefall around the Earth.
Scientific Basis: While the engines are firing, the crew experiences high G-loads because acceleration is physically driving their bodies into the couches. After Main Engine Cutoff (MECO) and Second Engine Cutoff (SECO), the spacecraft achieves an orbital regime. Here, weightlessness is experienced not due to a lack of gravity, but because the spacecraft and the crew are falling together at the same rate. The couches are oriented to ensure the body absorbs these G-loads in the optimal "chest-to-back" direction.
Current Limitations: The human body is highly susceptible to rapid fluctuations in G-forces. Therefore, modern launch vehicles have strictly programmed thrust profiles and trajectories to ensure they remain within the boundaries of safe, short-duration human physiology.