leak under the bottom of the pan when flushing "ONLY"?

It was me who answered you before and I confess I was confused by your expression about leaking from under the rim.

If the water comes from under the rim of the toilet pan then that is not a leak - it's an overflow - too much water in the cistern and the excess water runs from under the rim into the WC pan. You adjust the water inlet valve inside the cistern to shut off sooner - lower the float into the water.

If the leak is from between the two units then you may be able to fix it by just tightening the two wing nuts at the back - one on each side - you will have to get onto your knees to do this.

Whichever of these problems you have you will not need any special tools.

And if you still can't follow my suggestions then the services of a plumber might be best

The water you see under the bottom of the pan (on the floor?) may well have come from the join between the two units.

There is no need to thank me for twice going out of my way to try to help you. My time is my own.

Good Luck...
July 2016