Internal tension is something to seriously consider during logging, milling, drying, and during end use construction. It is the most dangerous in the beginning, and progressively less dangerous as the pieces get smaller and smaller through the cycle.
For example, cutting a storm tree that was twisted and broken off by a tornado, even with no other external signs of damage, can be very dangerous.
Having one of those slabs pop open shows the stress the log is under and i would take the advice already mentioned to make two qsawn boards.. though make sure and cut out the juvenile wood, or otherwise you will have crook in your boards once they dry. Ive had several 8/4 boards that showed signs they were under stress and wanted to come apart so i tapped them on the end with a mallet and blunt edge and they fell right apart.
You can rotate the piece and cut opposite sides to help even out the stress during milling- essentially keeping about the same amount of material on each side as it gets smaller.
For wide pieces , I sometimes rip cut them into narrower boards to relieve stress and either flip every other one and rejoin them or keep them the same orientation if you need to keep the grain pattern.
If your slabs are popping apart now in the green stage, expect that to exaggerate during drying. people try to clamp pieces together but that's a lost cause because you are unnaturally misshaping the cell into an oval and it wont return to a circle, even after adding moisture, so later during drying you get cracking.
It is also a consideration to look for ring shake before milling. Those logs can separate rapidly into large pieces and you dont want surprises like that!
Ive been cutting kiln dried pieces of oak with a handsaw while building a piece of furniture and had a small and loud (explosion?) of tension release where the board violently broke apart all the way down and fell in two with dust and splinters going everywhere.
Fine Woodworking (i think) had an article where the guy said he would drop his boards on the ground repeatedly to relieve stress.
Boiling, as mentioned, and also I feel microwaving both relieves stress in a piece.
So there really isnt a stage where internal stress isnt a significant factor. Usually you are better to avoid those once they show signs of stress. There is a reason lumber is sawn from the main trunk of the tree, and not the limbs.
Hope that helps . Good luck
