Information from Harry Harrington site: bonsai4me. This is a very useful article so I decided to share with you what I found.
During the Spring, the buds that you have retained will elongate into candles, their size being in relation to each candle's individual strength and vigour. To ensure that growth extension is regulated and energy is dispersed evenly throughout the whole tree; very strong and vigorous candles need to be pinched back.
Though the principle of Pinching Back Candles remains the same for Pinus mugo as it does for other Pines, the timing and technique does differ.
For Mugo Pines, if at any time a candle reaches approximately 1" to 1.5" in length and is noticeably longer (and therefore more vigorous) than other candles on the tree, gently break away the top of the candle with your fingers. This will stop extension of the candle and redistribute the energy of the candle to weaker areas of the tree.
If new growth is required to extend to fill out a branch or strengthen a weaker branch; leave the candle to extend without pinching.
Mugo Pine Summer Pruning for Backbudding and Ramification
The technique for increasing the ramification (foliage density) of a Mugo Pine and prompting backbudding is very straightforward. In Spring, the number buds at the tip of branch (the tip of last year's growth) have been reduced as previously described and begin to extend into candles.
During the Spring and early Summer (the exact timing will be in accordance to your local climate), the first flush of growth extends and the new fresh-green needles emerge. This seasons first flush of growth is much brighter and softer than last year's hardened growth.
If these new shoots are left alone, they will simply harden off through the remainder of the year and will not produce new growth until the following year.
However, if the new shoots are cut back hard in early Summer, the tree will be forced to produce a second flush of growth within the same season. Having already had to produce one set of new growth, the tree will have limited resources for this second flush of growth and the result will be very compact growth with short extension and short needles; ideal for developing the foliage mass of a Pine bonsai.
The exact point that the first flush of growth is cut back to depends on the strength of each branch. In strong, apical areas of the tree, the growth can be cut back to just two or even one set (pair) of new needles. In weaker areas of the tree (inner and particularly, lower branches), cut the fresh growth back to 3 or even 4 sets of new needles. Leave new growth that has only marginally extended and is very weak; this includes any shoots created from backbudding in the previous 2 or 3 years.
Never remove all of the new growth on any branch. It is important that at least one set (pair) of new needles remain.
This hard pruning of the first flush of growth prompts strong back budding in Mugo pines.
It is at this point that needle-plucking is carried out on a Mugo Pine. However, unlike Japanese Black Pines, Mugo pines backbud from where old needles remain on a branch not where they have been removed (plucked). So, whereas Japanese Black Pines are needle plucked to prompt back budding, the older needles of a mugo are kept in areas where backbudding is required.
Only remove the needles from the areas of last year's growth, where you don't want the tree to backbud; particularly along the underside and the top of the branch.
The result of the hard pruning of the first flush of growth will be that the tree produces a second flush of buds, some of which will be further back along the branch. With dimished resources in the tree and a shorter period of the growing season left, the second flush of growth will be shorter and more compact with smaller needles (not shown).
Any new back budding that occurs further back along the branch will be relatively fragile and dislodged easily. Take great care of them until they strengthen over the following 2 or 3 years. Wait at least 2 years before attempting to wire or prune these fragile buds."
Below informatins from: http://www.bonsai4me.com/AdvTech/ATPine%20Pruning.html
"Diagram showing vigour areas of a Pine tree:
Area 1 is the most vigorous.
Area 2 has medium vigour.
Area 3 is the least vigorous area."