The head tube on a bike is a short section of steel, is very stiff and very strong, and usually survives heavy impacts while the tubes behind it are what bend and buckle... what the tool does is straighten those tubes and this is where I become concerned with such repairs.

The head tube can withstand some pretty intense force and works as part of the lever to straighten the top tube... if it was really bent inserting the tool into it would be problematic. If you really bent the headtube the frame behind it would be totalled from the force required to do this and no repair would be possible as pretty much every tube and joint would be compromised.

For the most part, a frame that has bent behind the lugs is not one I would consider ride-able and even that lovely Paramount appears to have a crease behind the head tube that should be of some concern... once straightened the best approach would be to replace the top tube and down tube to ensure there is no compromise in the weld. The tubes may be straight after the process but the stress on the lugs and joints has to be considered as well as looks will deceive you.

The frame will probably be twisted as well.

If the bike was for personal use and you did the work then one would assume the risks in riding the frame, as one subject to liability issues I send my frames to a professional frame builder who has been doing this for 3 decades so he can assess them as to their repair-ability.

We have this elusive tool at our co-op and at another bike charity I volunteer at... they collect dust and I would not advise their use.