Classical astrology considers a Cazimi planet is extremely powerful and strong. This differs a lot from when the planet is conjunct the Sun within 8½° of orb, when it is said to be Combust and is said to be weaker in strength.
The strict measurements (0:17 and 8:30) are believed to be related to the visibility of the planet, which becomes blotted out by the bright beams of the Sun. When the planet is invisible because it's overpowered by the Sun's rays, it is considered disadvantaged.
By contrast, a Cazimi planet means "in the heart of the Sun," It thereby becomes fused with the energy and power of the Sun.
Some astrologers believe a Cazimi planet should be not just conjunct but also parallel by declination. Other astrologers question the validity of Cazimi strength altogether.
Although being Cazimi doesn't guarantee that the planet will have a positive expression, being Cazimi may give it an extra boost to make the best of whatever situation it is applied to. (This is discussed in a wonderful article by classical astrologer, Deborah Houlding, in the December/January 2014 issue of Mountain Astrologer magazine.)