Spending {T} allows you to activate the tap ability of a creature that's already tapped. Normally, you couldn't pay the cost of tapping the creature because it's already tapped. With Sole Performer, you don't have to worry about that. You have {T}!
You can't spend {T} to have a creature attack without tapping. You also can't use it to pay the cost of an ability that taps a creature without using the {T} symbol. For example, you can't activate an ability that says "Tap an untapped creature you control: [Do something]" with {T}.
For example, say you control Prize Wall, which has "{U}, {T}: You get {TK}." You activate Sole Performer to get {T}{T}. You then activate Prize Wall, spending {U} and one of the {T}s you got from Sole Performer. You could then activate Prize Wall again, spending another {U} and the other {T}. Prize Wall itself is still untapped at this point. You can then activate Prize Wall a third time, spending a third {U} and finally actually tapping Prize Wall.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Anna Christenson
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Spending {T} rather than tapping the permanent doesn't let you activate abilities of creatures that have "summoning sickness." That is, if a creature's ability has {T} in the cost, and it hasn't been under your control since the beginning of your most recent turn, you can't activate the ability (unless the creature has haste), even if you intend to spend {T} rather than tap the creature.
Although {T} acts in many ways like mana, it is not mana. An ability that triggers from a creature tapping to add mana wouldn't trigger when you activate Sole Performer's ability, for example.
Sole Performer turns {T} into a resource you can add, much like mana. You spend {T} to pay for that symbol appearing in the costs of activated abilities. If you spend a {T} you have, you don't have to tap the permanent that has the ability.
You lose unused {T} as steps and phases end, at the same time you lose unspent mana.
