No, "lifting off" is not the same as "popping up" - these are terms that are hard to define in English, so have always caused difficulties.
(Dave - this might be worth a FAQ
)"Popping up" is a term that is declared after a successfull activation at the start of movement. The Skimmers are considered to have risen high off the ground - possibly several hundred metres - allowing them to clear high obstacles like hills, trees, buildings etc. It is in effect a short flight during which the Skimmers do not touch down until the end of their action. (Note, this is still much lower than aircraft fly.)
"lifting off" is a much more limited term suggesting the Skimmers rising a few meters off the ground to get out of CC range, and this effect only lasts for the duration of the assault. As such "lifting off" and any subsequent movement is subject to terrain rules.
The distinction between these terms is important during an assault.
- Skimmers that "Lift off" are still subject to terrain rules, so may not enter Impassable terrain and must take DT tests if they start in, or move into / over Dangerous terrain. If the skimmers are carrying troops, these may disembark before the skimmer "lifts off".
- A formation that declares it is "Popped up" ignores terrain so may assault enemy located in terrain that it would otherwise consider Dangerous or Impassable. This is because the formation only "pops down" at the end of it's action, *after* it takes it's consolidation / withdrawal move. Note, it will take a DT test at that point if it lands in Dangerous terrain.
However, troops inside the skimmer may not disembark once it is "popped up", so if Eldar in Wave Serpents declare an Engage, they must stay "popped down" for the transported troops to get out after the 'charge' move - at which point the transports may "Lift off" to use their FF.
