Let’s say Corbyn gets 49% on first preferences (Burnham managing to get a few first preferences from him). If he gets a few Burnham second preferences, he will win. Corbyn-realists switching to Burnham must ditch Corbyn altogether or not bother.
Assuming Corbyn gets less than 50% and no second, third or fourth preferences, it’s between Burnham, Cooper and Kendall.
Kendall, the Right’s candidate, is the least likely. Assuming Burnham gets plenty of Corbyn’s second preferences, and a few of Cooper’s, Burnham will win.
The only way for ABC-Cooper to win is to get plenty of first preferences and second preferences from Kendall and Burnham, hoping that Burnham doesn’t attract too many Corbyn second preferences. The Cooper strategy is either 1. to attack Corbyn and Burnham, but without making him attractive to Corbynites or wrecking her chances of getting Burnham’s second preferences. That’s tricky. Or, 2. she attacks Corbyn and Burnham, in the hope that she can get 50% through first preferences and Kendall’s second preferences; very unlikely.
So: The ABC-Burnham strategy is either to attack Kendall and Cooper to get close to Corbyn and his second preferences, but without inducing Burnham’s supporters to give Corbyn their second preference. Or, he lets the others attack Corbyn, stays out of it, gets a few Corbyn votes, hopes his voters have Cooper as second preference, and hopes for Cooper and Kendall’s tactical second preferences to get him over the line. This ABC strategy is Strategy SPB. It relies upon no second, third or fourth votes for Corbyn.