From WoWWiki

Slice and Dice is a rogue ability. Currently two ranks:

• Rank 1 (At Level 10): "Finishing move that increase melee attack speed by 20%..."
• Rank 2 (At Level 42): "Finishing move that increase melee attack speed by 30%..."

Both ranks last for 9 seconds when using 1 Combo Point with an additional 3 seconds for every extra Combo Point past that. The "Improved Slice and Dice" Talent increases the duration of the buff by 15% per each of the possible three talent points.

The phrasing of what the ability does is ambiguous, but it is believed[1] that such melee attack speed increases are applied as follows:

SnD_Speed = Weapon_Speed / (1.0 + SnD_Buff)

e.g. for a 2.80 listed speed Weapon and Rank 2 Slice and Dice

2.80 / (1.0 + 0.3) = 2.80 / 1.3
                   = 2.15382 sec

Note: The above discussion is correct. For a more thorough discussion of increased weapon speed, see Haste