How to perform the Skill Point exploit in Ultima Underworld 1?

According to some commenters, and this wiki entry, there is a way to abuse the Silver Seed tree in Ultima Underworld 1 to accumulate additional Skill Points beyond what you normally can accumulate in the game.

Does anyone know how this works? When I hit lv. 2 in the game, I tried losing some XP, but even when I killed myself multiple times and brought my XP down to 1, I remained at lv. 2. Later, I hit lv. 15, and was just shy of 9000 XP. Then I killed myself multiple times until I only had 300 XP left, and then killed enemies until I was, again, just shy of 9000 XP. However, I didn't gain any extra skill points.

Is there something I need to do in order to lose XP *and* lose a level in the game in order to take advantage of this exploit? Or has this exploit been patched in the GOG version of Ultima Underworld since that post was written?