1. The Collective: This is the obvious one. In Advanced Mode the collective goes from auto-centring to incremental. This gives players finer control over their altitude and speed and makes it easier to control the approach for a landing, as well as to maintain level flight at lower speeds (without ETL causing a constant increase in altitude).
2. Effective Translational Lift (ETL): This is something that applies when you are moving faster than 16kts / 8m/s. You receive bonus lift, up to a maximum of 25% extra, the faster you go, up to 45kts / 23m/s, at which point the bonus is capped no matter how much faster you go. In Advanced mode, this bonus applies all the time. In Simplified mode, this bonus is reduced when the player lowers the collective. At 0 collective, they receive 0 ETL. The benefits to a Simple mode player are that they can hold the "down" key and descend at a faster (and consistent) rate, while still getting the additional lift when taking off from an LZ/doing a strafing run. This also makes the approach for landing more predictable and constant. However the big trade off is that without that bonus lift, when the Simple mode player starts taking engine/rotor damage they will find that descending is suddenly a lot faster than it used to be and if the engine gets completely knocked out, they will not be able to autorotate effectively.
3. Ground Effect: Advanced Mode uses it, Simple Mode does not. So, what is Ground Effect? The simplest, scientifically-inaccurate-but-good-enough-for-here explanation I can give for Ground Effect is that it's a phenomenon where the close proximity of the ground interferes with the downwash airflow. Normally that airflow would cause lots of drag on the rotor, but when it's messed up by the terrain, that drag is reduced considerably, which upsets the lift/drag balance in favour of lift. i.e. You basically get more lift for the same amount of power and collective. My in game handling is based in reality, but the calculations are very simple, to get the general effect without all the complex physics. Basically, if your altitude is lower than 1.25x the diameter of your rotor, in whatever helicopter you're flying, then you get a boost to lift. That boost is calculated on a curve. The max boost you get is 30% extra, and that applies at an altitude of 0. At an altitude of 0.25% of the rotor diameter, that boost has already dropped to half of the max (i.e. 15%). At half the rotor diameter, you get a little under a quarter of the max and at three quarters diameter you're all the way down to 10% of the max. That drop continues all the way to 1.25x diameter, after which you get 0 bonus lift.
Advanced players however can use this to fly at low altitude on a greatly reduced collective. It also provides a cushioning effect when coming in to land, helping slow the helicopter incrementally as it approaches the ground, which allows Advanced players to descend more rapidly, but then to use a combination of a late flare, ETL and Ground Effect to arrest their descent rapidly and still land softly. When combined with ETL, it's also a key component of flying while damaged. A combination of rotor and engine damage can reduce the max lift to levels where the helicopter cannot fly. However in Advanced mode, Ground Effect can boost the lift enough to get airborne a meter or 2, then with some gentle acceleration, ETL allows the helicopter to gain altitude and fly away despite being too damaged for a Simple mode player to move. The Advanced player can then return to base for a repair.
