The additional effect of Flee will proc according to the level of the mob you are fighting in relation to your own. If the mob is of a lower level to you, the effect will not proc. If the mob is a higher level than you are, the effect has a higher chance of proc-ing.
Is this the fastest weapon in the game? With nin you could do 1 hit with 132 delay. Thats with this weapon thats some fast as hell tp build, in a 20 second fight you get off 15 hits.