The language there is confusing, but I'm pretty sure it's just stating that when temporary hit points are lost (such as the duration of the effect giving them to you expires), your hit point total doesn't drop because of that.
For example, you're 10th level and use Minor Shapeshift with Polymorph fueling it. Now, in combat, you lose all of your temporary hp through absorbing damage and now you're sitting at, say, 15hp 4 rounds after activating Minor Shapeshift. The duration expires, but you have already lost all of your temporary hp. You're still at 15hp.