The effect isn't absolute. The "friction" of the jetpack is something like '5' which means slowing down from default g_speed * 5 per second meaning 320*5 = err.. dnt have a calculator on me. Anyway, "floor friction" is '6' so thats why it seems pretty much instant. If you turn on and off ur jetpack just before you land for only 0.1 seconds, it isn't guaranteed to save you from taking damage.
Also, you can 'float down' to safety with a jetpack without a reactor. All the absence of a reactor does is just simply turns your jetpack back off. Hence, you can spam the 'on' button. I have my 'g' key bound to itemact or itemtoggle jetpack and i just rapidly press that button before i land. It's simple. If you have the old tremulous 1.1 client, you can also just 'hold the button down' and you'll float down slower.