 Hot Tapping or Trepanning has four steps, as shown in the video below:
First: Â A tie-in with flange is welded (or fixed with a collar)Â on the existing pipe, with the diameter and angle projected for the new branch and a full open passage valve is installed on the flange.
Second: The HT machine is set on the flange of the open valve, it is pressurized and a pressure test is performed ( water tightness of the setup). Drilling starts. The cut is done without fall of the shell,or coupon, into the pipe, retained by a tool lock.
Third: The cut is finished. The machine advance is reversed to carry the tool and the coupon back, out of the valve, which is then closed.
Fourth: The machine is withdrawn after a controlled  release of excess pressure. The new branch can be installed and the valve opened to let flow start.
 Know a bit more:
