As this is a new, unstable medigun, it slowly switches through different resistances of:explosions,bleeding,melee,bullets,fire,crits,minicrits,mad milk,etc. it does this by itself.
+current resistance type gives 50% resistance to that type of damage or effect.
-50% heal and overheal build rate
-25% max overheal

Ubercharge causes the abilities to switch at lightning speed, causing it to give you 50% resistance to ALL EFFECTS, DAMAGE, KNOCKBACK, ETC

Also as a cosmetic effect, the little turn wheel on the backpack turns every time the effect changes, and spins rapidly during uber