The clast is rotating as it's being carried along in the water, and if it hits the substrate with its edge pointing down in the downstream direction, the clast will embed.
So when you're looking at a rock with embedded clasts in it, you can determine the direction of current by measuring the maximum down dip direction on the clasts.
In other words, if the clasts are all dipping down in a certain direction, that's the direction of streamflow when the rock was deposited.