Difference between blocks and decelerates
 

I am not able differentiate between the following sentences:

1. The vortex blocks the flow.

2. The vortex decelerates the flow.

I understood the second sentence. It says that the vortex decreases the flow velocity.

But I am not able to interpret the first one. When a vortex or some body blocks the flow does that mean the flow reverses it's direction i.e., if the flow is occurring say along the positive x direction, if the flow is blocked, does it start to flow along the negative x direction?

What is the difference between these two sentences?