Transformation thinning has several aims:

To retain the maximum species diversity
To create more structural diversity of stem diameter and height within the existing crop.
To improve the quality of the existing crop by removing poorer quality trees.
To improve the quality of the future crop by improving the growing conditions for the best quality trees.
To reduce the overall basal area of the crop by comparison to a traditional plantation.
To encourage the survival of any advance regeneration and facilitate its recruitment into the upper storey.
To gently manipulate the stem diameter classes towards a reverse j-curve.

These aims are realised by:

Selecting “future trees” and identifying them with green spray paint.

The selection of a “future tree” is based upon its species, its quality, and its diameter.

Do we need this tree to perpetuate its species into the future crop, either for itself or its seed?
Is it of sufficient quality to merit its retention, (its retention on the basis of its species may over-ride its lack of quality).
Do we need to retain trees of this size class to create the desired stand structure?

Marking trees for removal.

The marking of trees for removal is driven by the following:

Is it competing with a future tree?
Is it of a poorer quality than its neighbour?
Is it of a size class that is over represented in the stand?
Is it of a species that is over-represented in the crop?
Is there advance regeneration that’s needs to be released?
Do we need to remove it to help reduce the overall stand basal area?

The marking of trees for removal is constrained by the following:
What effect will the felling process have upon its neighbours?
Will the tree significantly increase in value if left?
What effect will its removal have upon the stability of the crop.

When dealing with larger trees closer to maturity the question of value becomes more important. If the tree is of good quality and will continue to increase in value if left then it should not be felled. (unless you need the money it will realise at this time) If the tree is of good quality but will decrease in value if left then it should be felled. If the tree is of poor quality and will not increase in value if left then it should be felled. These decisions can be formalised by setting target diameters for the various species. (the target diameter is the diameter at which the combination of the volume and the unit value make the tree at its most valuable) However at this stage in the transformation other factors may be more important than target diameters. Target diameters also have to be flexible to reflect fashions in the market place.

Regeneration

If you do all of the above, seeding and regeneration will usually follow and the question that needs to be addressed is what prevents the seeds germinating and becoming viable saplings. The first is predation. Seed and seedling predators must be kept at a level, either by control or exclusion, which allows regeneration.

Second is the condition of the seed bed. As the seed germinates it must be able to quickly get its roots into mineral soil. If the leaf litter is deep this may require some scarification. Third is weed competition. This is mostly down to light management, if too much light reaches the forest floor weed growth will out compete seedling growth, if not enough then seedlings will not survive. However in the early stages of transformation herbicide use particularly to control bracken or bramble may well be required.

Whilst it is preferable to use natural regeneration there is a case for some planting. At the beginning of the transformation natural regeneration can be slow to come so to try and avoid too long a gap between the existing crop and the next generation then some planting after the first and second transformation thinning is acceptable.
If you are looking to introduce species that are not present in the present crop, or to improve the provenance of future crops it is also acceptable to plant the required trees. If you have a problem with predation, then planting and protecting a few trees, can also get the transformation process going, allowing extra time to bring the tree predator numbers under control.
It is interesting how natural regeneration will often follow two or three years after planting.