A few weeks ago I posted some eagletree data and noted the current being put back into the battery in the second post of this thread.
http://www.rc-monster.com/forum/showthread.php?t=18458
Regenerative breaking is very simple the way I see it. If you recall that motor torque is proportional to current, and that torque is negative during breaking, your are going to get negative current, or in other words, current that is charging the pack. When you charge a pack, the voltage increases.
If it doesn't make sense how a motor would create negative torque on its own without using power, think of what happens when you physically try to spin a motor faster than its kv*volts. It begins to push against you. You can drastically reduce the kv of a motor just by adding a resistor or equivalently, switching the esc on and off the same way it switches with different throttle amounts. This is just a guess based on general knowledge of circuits. I don't know the specifics.