Hi!
Like Rainboe, Bart and others have said: Ask the instructor what he means and whats the purpose of the exercise.
I've only flown gliders so my experience with the C172 is shall we say slightly limited.
I've done quite a bit of spinn training i various gliders. One common thing with all of them that has a trim tab (like the C172) is that they are more willing to spinn if you trim them nose heavy instead of tail heavy. The reason for this is quite simple. While trimming the plane nose heavy, the elevator is more effective pushing the tail down. This gives us a greater AoA with the stick at its rear limit then if the plane was trimmed tail heavy.
When flying gliders a situation with the CoG behind the rear limit can occure in two cases:
1. The ground crew forgot to remove the tail dolly (removeable tail wheel). And you forgot to check it before takeoff.
2. Flying with waterballast in bot the wings and the tail in to cold weather. When dropping the water the water in the tail will freeze up before the water in the wings.
The solution in the first case is to land as quickly as possible, and keep your speed well above the stall speed. You can easily notice during takeoff that you have the dolly on.
The other cas is more subtile. It's nearly impossible to notice. But if you suspect that the water in the tail have frozen (and you have let go of the water in the wings). Then again keep the IAS well above stall speed at all times.
Best regards
Bernie