Horses that know how to change a lead and then only change in front are having rider problems. Either the rider is trying to change with a direction change (instead of on a straight line) or letting the horse's shoulder fall in or down on the new lead side.
For a good reining change, the horse must stay collected with its weight shifted back where it belongs. The horse cannot be heavy on its front end and it cannot drop its shoulder or fall into the new direction.
The reason you do not see reining trainers use a log is the same reason you do not see a dressage trainer use one. You get a 'hunter change' where the horse is not collected and the horse changes in front first, hopefully catching up behind. The log helps them catch up behind.
Once a trainer learns how to keep a horse's hind end under him and keep his front end up and learns how to teach hind end control, that person just does not have lead changing problems.
Let me go into more detail about hind end control. It DOES NOT count as hind end control (and does not work) if you take a horse's head in the opposite direction as you are trying to push the hind end over. You need to have a horse obedient to the 'diagonal aids' which is much more difficult and much more advanced than having a horse obedient to the lateral aids.
'Leg yielding' exercises where the rider applies the right leg and shortens the right rein and moves the horse to the left WILL NOT prepare a horse for a lead change. The same is true of taking a horse's head to the right while the rider's right leg pushes the horse's hip to the left. This will never get the horse bent the correct way or get his hind foot under him for a complete change.
A proper 'half pass' where the horse's head is looking the direction AND the horse's hip is going in that same direction of the lead you are going into is what is needed for a good lead change. This is a very difficult maneuver, but trainers that have no problems with lead changes and have horses seamlessly move from one lead into the other in a straight line without hopping, speeding up or changing cadence and stride, do it this way.
They use reverse bend or a counter canter. When they change legs and leads, the horse moves into the position they have learned by doing a proper half pass at the lope. This way, their head and hip are both moved into the new lead. Collection is always one of the most important ingredients. The lead change should always come on a straight line and never when changing directions.
This is pretty difficult to explain over the internet. I hope this in not real confusing. This is where almost all the people I know have had problems with lead changes.