The alternate bat has the following characteristics:
It is an M type structure (like the gartley) that utilizes a 1.13 XA extension rather than the normal .886 XA extension you would find in a bat. The BC extension ranges from 2.0 - 3.618, but it is normally a 2.618 BC or 3.14 BC extension. Another major character of the alternate bat is an initial retracement of the A leg to .382 (a bat normally has a .50 A leg retracement). I hope this makes sense.
Here is a link explaining the pattern ---> http://theresponsibletrader.com/trt-ichi-monics-the-alternate-bat-pattern/