Can someone explain Momentum to me?

What's this deal with Momentum's swiftness buff? Yeah, it's 1.5 seconds of move speed. I also see that my momentum stacks do not appear to increase during the time, but is still present on the buff bar.

What value of momentum stacks is it at for those 1.5 seconds? Am I getting the full 5-stack value, or 1-stack, or 0?

You need at least 3 stacks of momentum to get the swiftness buff. If you move before you reach 3 stacks, you get nothing.

Its great for stick-n-move builds.

from the Wiki page:

Momentum Support is a support gem. When you use the supported attack skill, it grants a stack of Momentum, granting increased attack speed per stack. Momentum is removed when you move while you have it. At five stacks, Momentum is removed to grant the Swiftness buff, which grants increased movement speed.

No. Specifically when the swiftness buff is activated, what attack speed value does the gem have?