The video starts off by demonstrating that everything works as expected in both the track effects section and the sampler fx chain when a phrase is not making use of the the sample number column.
0:32 - The phrase is switched to an identical version with the sample number column enabled. Now the Velocity Tracker in the sampler fx chain only responds to changes in the velocity column when they’re being used on the same line as an actual note. The exception to this is sample 0, which responds as it should.
1:13 - When the Velocity Tracker in the track effects section is set to respond to a specific instrument number, it instead responds to velocity changes from the numbered samples in the phrase.