If you swim across a flowing river you point perpendicular to the river's axis and allow yourself to drift downstream. If you aim to swim to the point on the opposite bank... you end up drowning.
Thinking of the situation where the speed of flow of the medium in which you are travelling (ie the windspeed or the river's flow) is a large proportion of your speed or that of your vehicle (airspeed or swimming speed) will help clarify the effects.