Think of the Christmas season. Does it feel more like Christmas on December 20th, or December 27th?
The 27th is closer in time to the day itself, but Christmas is separating: the day has come and gone. On the 20th, the anticipation is in the air, holiday parties are happening, Christmas season is in full swing. We are applying to Christmas at that time.
Aspects work like that. The time when they're applying is when the aspect is being anticipated. It's like that Christmas-is-coming energy. The time when they're separating is when we may still be in the holiday period, but we can feel it passing.
Because they work that way, applying aspects will manifest strongly at a wider orb than separating aspects will. Just like the December 20th/December 27th contrast. It feels even less like Christmas on December 29th, doesn't it?
But a close separating aspect is still going to be very strongly felt. If it's within a degree of exact, it's like lunchtime on Christmas Day: the most anticipated part may be over, but the celebration is still going on.
In a natal chart, applying aspects are a gift that's in the process of being given or a problem in the process of being created. They manifest gradually over the native's life.
Separating trines and sextiles are a gift that's already been received and opened. They manifest full blown early in life, like in
this case (full disclosure: that's a letter I answered on my blog).
Separating squares and oppositions are challenges that are more likely to become easier over time, because the full challenge has already manifested and started to fade. The applying square or opposition is kind of like, you inherited a forest cabin when there was a fire raging around it. You're in the thick of it. The separating square or opposition is more like inheriting that cabin after the fire, so you get the task of repairing and rebuilding and stewarding the land as it regenerates.