Benefit: Whenever you are adjacent to an ally who is raging, you may choose to enter a similar but less powerful rage as a free action on your turn. This weaker rage gives you all the benefits and penalties of a barbarian's rage, except your morale bonus to Strength and Constitution is only +2. There is no limit to how long you can rage, as long as you remain adjacent to a raging ally (for example, you could take a 5-foot step away from one raging ally toward another raging ally and maintain your rage). As with a barbarian's rage, when this weaker rage ends, you are fatigued. You cannot use this feat if you are fatigued.
As soon as the sympathetic rager is no longer adjacent to a raging ally, your rage ends. If the raging character take a 5-foot step away and no raging ally is adjacent to her position, the rage ends. If the raging character moves 30 feet along a line of raging allies and every square in that movement is adjacent to a raging ally, then the rage continues.
Since Pathfinder, regrettably, has no support for coordinated simultaneous movement with allies*, there is no way to advance at typical movement speeds using this feat without a lot of ridiculous setup. You're better off activating this feat once you are already in melee and aren't expecting to have to move much.
* (edit) Well, I suppose one rager could ride the other, if the GM decides that "occupies the same space" is good enough to mean "adjacent".