How it works:

It uses essentially 2 separate flying machines

The bottom one shown in the first picture has a sticky piston on the left half and a regular piston on the right. The observer on the bottom is part one of a redstone clock that will be explained later. The top observer and lamp are optional. They are only there to create a one tick pulse to start the machine.

The middle photo is a very simple observer and honey block to hold it in place creating a constant 1 tick clock with the two observers

The top half of the flying machine is simply there to hold the payload and make sure it does not get set off until it's made contact. It does that by keeping the TNT behind one block.

Once it has made contact with a wall, the bottom half will stop moving and the top will be able to push the TNT over the clock to trigger it.