Upon being successfully struck by a foe, a character subtracts the damage done from their current hitpoints. There's nothing unusual UNTIL your HP reaches 0 or below. Once this happens, you make what is known as the famous "Willpower Check". A 'Check' in Spherus is always made on a d100, as are many other rolls (such as the To-Hit roll). To succeed at the Willpower check, you must roll equal-to or under your Will-power on 1d100. Which, if you'll notice, at first level is very difficult! If you succeed, you are still conscious, with 1 HP left. If not, you fall unconscious. How long you are unconscious is usually up to the GM's discretion. If you were only 'under' by 1 or 2 HP, you'll wake up quite quickly. If you went over by a fair bit, you may be bleeding to death and won't wake up at all without special attention!

Note that you may only make one such will check upon being reduced to less than one hitpoint. Any further damage immediately knocks you out, and can still lead to the following special case.

There is an exception to the above. If you take so much damage that it would reduce you to less than your negative Max-HP, you are killed instantly, with no will check or anything! Maybe that sounds a little confusing. Say that you have 12 HP (you're at full strength). Your negative-Max-HP is -12. If you take 25 damage from a single blow, simple math shows that it would reduce you to -13 HP - which is LESS than -12, therefore, it kills you immediately! If you only took 24 damage, you would be at -12, and would only be unconscious (if you failed your will check), albeit in pretty bad shape.

Psi-points are much more simple. All Psi-Xaquel moves and some of the more powerful Art and Weapons moves take PP to perform. If you don't have the necessary PP, you can't do the move. Simple as that.