On to the subject:
A chart dispositor is a planet that 'rules' your chart. The energy tends to influence the rest of your chart's behaviour(s) - (influencing other planets). The chart dispositor can be recognized by looking at the sign on your ASCENDANT. If you want to find out what your ASC-sign is, you can go to AstroDienst and calculate your own chart.
My own chart dispositor is Saturn. So all the features of Saturn are strongly visible in my personality. A more serious nature, reserved, disciplined than for instance Jupiter as the chart dispositor. It's a part of establishing a general overview of the radix (official term for birth chart) before going into detail.
But where it really gets interesting is watching your dispositor progress through time on your birth chart. This is called a 'transit or transition'. Saturn is usually feared by many because his keywords are not as happy and jolly as his counterpart Jupiter. In transition, when Saturn makes connections to your birth planets I have found that Saturnal people have less severe consequences if the transition itself is hard to cope with (for instance Transiting Saturn opposite Ascendant). Sometimes even, having beneficiary effects for the one born under it. The other way around I have found that for instance a person with a Jupiter chart dispositor will not react well to Saturn transitions/ have more painful situations coming at them.
I've never seen "yes, the dispositor needs to be specifically watched when making estimates in 'forecasting'" being spelled out in any astrology literature, but by watching transitions on my chart as the Saturnal entity and watching someone elses as the Jupiterian entity - this concept strongly comes forward.
Now, what if your chart dispositor is Mars (Aries on the ASC)? Mars is a fast transiting planet. My logic tells me that the effects of transitions are minimized for the idea/concept I just came up with. BUT! And here's where secundary directions come into my theory! A 'direction' in an astrological sense is another way to 'forecast'. There are a couple of types, but basically it's one day = one year. The exact meaning and the way you calculate it, I will leave behind. The only thing you need to know is that Mars is much slower in secundary direction. I will try and research if the secundary directions have the same behaviour on someone as transitions of the slower planets.
Note: Also known as Secundary Progression.
This is the concept in a nutshell:
Your chart dispositor does not only rub off on your character, it makes it easier to cope with it's energy when transiting/progressing. You are 'better equipped': to grab a chance, to keep your head above water, to enjoy.
If you calculate your own chart and find out your chart dispositor at AstroDienst you will see transitions in the 'forecast' area. If your dispositor is either Jupiter/Saturn/Uranus/Neptune/Pluto and is involved in heavy transitions you are now able to think about this concept. Feel free to post entries/ comments on this issue.