Hello,
Link Velocity is something like
Week 1: You build 1 backlink to your website
Week 2: You build 10
Week 3: You build 80
Week 4: You build 200
Its the increasing amount of backlinks you build to your site over a period of time. It must keep increasing so Google will detect the increased importance of your website and rank you high.
If week 1: You build 200 bls, week 2: 100bls, week 3: 10bls. Google will detect a decrease importance on your site and won't rank you as high as a website increasing their backlinks overtime.