This example works for me since I have experienced it myself. During fire fighting in a reasonable wind, you try to take off into wind and get at least 40 knots on the clock before turning out. There are times when this is not possible and as you turn downwind you really have to be careful because you lose IAS and the helicopter descends with the loss of IAS and sometimes translational lift in strong wind conditions. The helicopter also accelerates very slowly with a few tons of water hanging below and even though I have been aware of this, it has still scared me silly more than a few times.
The principle remains the same as Dennis has explained, the helicopter has inertia and when turning downwind, especially if the turn is very tight, it takes time to overcome the wind gradient and therefore the IAS decreases, possibly to below transition. Without the translational lift there is insufficient pitch / power available to overcome the rate of descent established in those few critical seconds and a prang is very likely as in this example.
I agree that you are flying in an airmass, there should not be a change in IAS and there wont be if the turn is gentle. With a really tight turn the inertia is much more obvious and there is definitely a drop in IAS when turning downwind. It wont make a difference if you started the turn with sufficient IAS, as Dennis said, but it certainly will if you start the turn with the speed too low.