I'll give an example:
lets say you have this rage,
level 1 rage:
Mad Dog rage:
str VS AC
hit: 3W+str
miss: half damage
effect: you enter the mad dog rage, you gain +1 fort.
so when bob the barbarian is in a fight against a bunch of goblins and he wants to enter his rage, he uses the daily power 'Mad dog rage' and makes the srt vs AC attack against an enemy. Hit or Miss, he then enters the rage until the end of the encounter, ie he gets the +1 to fort. His is considered raging for all powers and abilities that key off of raging. This rage lasts until the encounter is over, he enters a different rage, he drops below 0 hp, etc.
lets say he has another rage:
level 5 rage:
Angry cat rage
str VS AC
hit: 3W+str
miss: half damage
effect: you enter the angry cat rage, you gain +2 damage.
Now if Bob the barbarian is in his mad dog rage and wants to use his level 5 rage he has two options.
1) use the level 5 power as normal: he makes an attack for 3W+str, and then gains the +2 damage, but loses the +1 fort. he is still considered raging, but its now the angry cat rage.
2)He can use up the level 5 power with rage strike and deal 4W+str damage, but keep his +1 to fort and not gain the +2 damage. He is still considered raging and its still the mad dog rage.