These 3 Japanese Maples were originally garden tree’s but a borer attack seen them removed and used for Bonsai trainers .These were placed back into the ground last year and had some whips thread grafted onto the bases . Not all were successful , they were re potted during winter and have since been defoliated .
Now they are under going some more root work .I have used 2 methods here Thread grafting 
seedling and A tooth pick method for growing roots . Firstly both methods require a drill hole , however the thread graft cant always be placed where required due to the base being to big .This is where the tooth pick method has the advantage . Thread grafting is done by simply drilling a hole through the trunk or root , then a seedling that has been defoliated is inserted into the hole 
.The roots of the seedling is located in the required position and back filled with soil . .They are then allowed to grow on fusing to the original tree base , they are then severed from the top and the new root is created .The tooth pick method is used to stimulate and create a root 
where it is not possible to drill a hole trough the trunk base or in an awkward position ,Simply drill a hole where required to a depth of about 10-15mm . Then using a bamboo tooth pick snapped in half dip it into water , then into hormone powder and insert it into the drill hole .
This is then covered with soil and treated as a normal bonsai regularly watered .Over the growing season a new root is produced in the required position .In some cases a combination of both methods has been used to do the job .
These trainers go through a series of defoliation’s during the season .As many as 5-6 defoliation’s can safely be performed on healthy tree’s if the growing conditions are ideal .I have pushed Japanese Maples 7 times in a season however they have died due to the stress of the exercise , now i do 4-5 or 6 depending on the tree. These have had a full defol’ and a second minor defol’ and selective trim .In around another 3-4 weeks they will again be defoliated after the new leaves have hardened off . This will be repeated until late Summer early Autumn then allowed to change their colour and drop leaves as per normal winter seasonal change .
Cheers Alpineart