You stride up to your Speed. During this movement, you can try to move through the space of one enemy. Attempt an Acrobatics check ... as soon as your try to enter its space...
Success You move through the enemy's space...
Failure Your movement ends, and you trigger reactions (such as Attacks of Opportunity) as if you had moved out of the square you started in.
Consider the situation where you start at A and use Tumble Through, stride to B, wanting to tumble through the monster at C to get to D.
|A| | |B|C|D|
I think that "The square you started in" is a bad choice of wording, since you start the action of Tumble Through in A, and you did in fact leave A, provoking as normal. If you failed the Acrobatics check, you END the action in B, so the failure line should be updated to reflect that.
Ya, the square you started part is confusing.
New question: success says nothing about avoiding reactions due to movement. Does tumbling through a fighter trigger a reaction?
Are there ways to use acrobatics to stride without triggering, or is that only possible with step and the feats that increase the distance of step?