You can put the opposite aileron in too fast and ms the correct amount to flat spin some aircraft... Take it VERY HIGH, enter the spin and SLOWLY change to opposite aileron. When it flattens, don't add more opposite aileron or you can pop it out into an opposing direction spin or spiral.

As you gain skill in doing the flat spin you will increase the rate that you put in the aileron... and you will eventually learn how to put in the correct amount right as the aircraft initiates the stall and wing drop to be in the flat spin in less than 1/4 rotation of normal spin. (looks like it just goes directly to the flat spin)

You can flaten any spin... just if the plane is too nose-heavy, it never flattens well. Trainers rarely have both wings stalled so they pop out of the spin when you flatten it.