It's like the orbs you use for aspects, if you use 10 degrees for an aspect, then you start to feel the influence when the planet is within 10 degree orb, our mind may begin to focus on the planetary issue, but nothing major happens yet as there is a slow build up. We feel it more the closer it gets, when exact the issues of the transit will be in full effect, and the transit fully leaves once it is 10 degrees away. This slow process gives us time to respond to the energy, integrate it and then to fully process the event/issues until it moves completely away.