Well, I'm drinking HARD with a few lads right now, so I cannot post ya any proof, m8 but read this copy from some old post:
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If you want to change speed during flight...say that the chopper needs to start out at full speed
and then slow down over a certain area, it's best to do it with triggers. If you do that with a waypoint,
the chopper will fly past the waypoint, do a turn around, and head back towards that waypoint
and then turn again and continue on it's path. Looks pretty silly in a mission. Setting it with a trigger,
the chopper will just do a slow down flare but continue on the same path at the new speed.
(extended rep. by stompmaster)