Don't forget that autopilot only have +/- 20% range of the engine power to maintain set altitude, so if you are in a break turn or flying into a mountain at 250 km/h... don't count on it too much or you will crash, pull the collective yourself in situations where radical changes in alt are possible. Also, don't forget that there is a switch of what altitude you want to hold - radar or barometric.
[next section is all AFAIR]
The other RL vs. DCS issue is much trickier. Imagine you have alt hold enabled. In RL to move the collective you must disengage the CB. So what happens is - you release the collective lever AND simultaneously disengage altitude holding by autopilot. When you let go on the lever - you engage AP AND set a new alt to hold for it.
In DCS it is possible to move collective WITHOUT disengaging CB (I guess 80% of virtual pilots do this). So if you have alt hold channel enabled then AP is still working and working against your movement of collective lever. And so if you, say, lowered your collective by 60%, you would, in fact, have the AP fighting against you with its 20%, so you will have 60% - 20% = 40% as it tries to hold the old alt. If you don't pay attention to it altitude dynamics of the chopper can be rather unpredictable, especially in NOE flight. :)