Hi,
There seem to be two views about what constitutes as 'chidra'. First and possibly commoner view is that it is the transition period when one mahadasha ends and another one begins. The premise underlying this is interesting. If we think of the vimshottari dasha, for instance, it is calculated based on the placement of natal moon in a certain nakshatra. This determines the dasha at birth. Let us suppose it is one of the nakshatras of jupiter. The entire span of the nakshatra of jupiter would represent the 16 years of Jupiter dasha as assigned in scriptural texts (such as B.P.H.S.). If moon is at a point in the nakshatra about 1/4th in it, then the remaining 600 minutes yet to be travelled (each nakshatra is 800 minutes or 13d 20m long) would represent 12 years of jupiter yet to be experienced by the nativity. Thereafter will come dasha of saturn (19 years) and mercury and so forth.
One can conceptualize a vimshottari dasha as the 'progression' of natal moon, from the point at birth through remaining lifespan. One can also appreciate that unlike western progressions which ascribe a constant velocity of progression, in dasha progression, moon progresses at variable velocities, covering the same span (800 minutes) in six years in nakshatras of sun, ten years in those of moon, sixteen years in jupiter nakshatras, and so on.
Dasa chidra, therefore can be visualized as the point where the progressed natal moon leaves one planet's nakshatra and moves into the nakshatra of another planet, next in the chain of nakshatras. The KP subs are also connected to this concept by extrapolating and translating the vimshottary bhukti into the KP sub.
Moon is the indicator of mind (manas) and as it changes nakshatras, the influence under which it operates changes and so adjustments in thinking, attitudes etc must change to remain in harmony with our astrological rhythm.
In actual observations in older people who have experienced a few dashas, there is not always a huge problem or even drastic change seen. In others, there is! Here comes the need for considering factors such as different sources of strength, placement and attributes of the dasha lords and a variety of ancilliary modifying conditions and factors that constitutes the general infrastructure and framework of jyotish delineations. In short, one size does not fit all.
If exploring this rather interesting facet of jyotish, one must remain mindful of the troublesome (potentially!) reality of choices of ayanamsha which can shift the dasha ending and next one starting (chidra or hole or gap or simply transitional zone).
Hope this helps you somewhat.
Regards,
Rohiniranjan