This is a good question and I would answer it the way I usually explain to my clients.
You are like a company, your mind is like the CEO office.
The Planet whose dasha is running would sit on the CEO’s seat and would run the company in his own manner.
During the period of the planet, a person not only gets the fruits of his Krama as the planet is going to give him (e.g. Yog Karaka Planets normally give good results, while troubling Planets normally give bad ones).
MOREOVER I HAVE SEEN THAT A PERSON’S NATURE REFELCTS THAT CHARACTER OF MAHADASHA RULER.
Assuming that none of them are particularly good or bad, and would give their natural results.
During Saturn Mahadasha: It’s all blood sweat and tears. They are this slow, unassuming, hardworking but ambitious person who works his guts off but often doesn’t get the desired results.
ENTER MERCURY: I often use this sentence “When Saturn goes and Mercury comes, the Tortoise becomes the Hare”. I’ve seen “average” students who barely passed despite the hard work during Saturn period suddenly “top exams” in Mercury period and get recognition. They make their careers and what not. Mercury dasa helps generally everyone think better and do well. They do really well in material world and are often recognized by others as genius.
ENTER Ketu: I often use this sentence “When Mercury goes and Ketu comes, the Genius loses his head”. The same person who was saluted for his genius suddenly loses his head because of some emotional or mental trauma that comes with a Ketu dasha starts. They virtually “lose their head” and brain fog comes around them. The go away from material world that turns harsher and harsher by the day and they gravitate towards spirituality.
Same person, three different Mahadasha rulers, three different lives. I often tell people Planets LIVE THROUGH YOU DURING THEIR MAHADASHA. so learn to be detached.