in an exposure mode that is not manual, you MUST lock the exposure, otherwise as you recompose, the exposure will vary wildly depending upon the reflective brightness of the target in the spot (in manual mode, the metering reading will change but not your exposure as you have full control over it)
normal spot meter
meters assuming the subject is an 18% gray card
if you expose according to the reading giving 0.0EV, the target will be rendered as 18% gray even if it is actually black or white - this is how camera meters work by default
HI spot meter
this is designed to be aimed at a target which you want to appear as white but not blown out
it is similar effect as using normal spot meter and using a meter reading of +2.3EV
LO spot meter
this is designed to be aimed at a target which you want to appear as black but not be blocked out
it is similar effect as using normal spot meter and using a meter reading of -3.0EV
to really learn to use the spot meter modes, learn Zone System first
the light meter does NOT KNOW how light or dark a person's skin is and will always get the metering WRONG UNLESS you adjust it properly:
for example once you have learned that a Caucasian face is in Zone VI and 1 stop LIGHTER than Zone V where 18% gray card is and where the light meter wants to put your face, you will realise if you meter of such skin, you need to use an exposure compensation of +1EV in order to render the skin tones accurately!
if instead, you meter off an almost black African face which is in Zone III, you need to set exposure compensation to -2EV.