Moving 25 yards without stopping grants the Monk +20% additional damage for 6 seconds.
Momentum is a Monk passive skill, unlocked at level 66.
This effect does not stack, but works regardless of which method the Monk uses to move: running with Tempest Rush, dashing with Dashing Strike and teleporting with Fists of Thunder or Epiphany count as well (if the Monk covers the required distance). Even being pulled by a monster's Vortex affix or being a victim of a Knockback will suffice. Once 25 yards are crossed, the count is reset, so the Monk must move another 25 yards without stopping to refresh it.
This buff is not multiplicative with other damage modifiers.