Trinity survived the dismissal of former NRL enforcer Emre Guler on the hour to claim a 26-16 victory and book a trip to either Leeds or Wigan next weekend with the grand final just 80 minutes away.
Second-half tries from Isaiah Vagana and Caius Faatili helped Trinity to establish a big enough lead, and afterwards Powell said: "It was a pretty helter-skelter last 30 minutes, but I thought we were awesome for large parts of the game then we saw it out when we needed to.
"I thought in the first half we were outstanding and we dominated the game, and the scoreline didn't really show it. Then at the start of the second half we just blew them away.
"Emre getting sent off changed the game a little bit, but we did some really smart things to see the game out."
The brace from Vagana and Faatili just after the interval settled home nerves after David Armstrong had halved Leigh's deficit out of nowhere on the stroke of half-time.
However, prop Guler inexplicably set up a frantic finish when he was shown a red card for punching Leigh counterpart Owen Trout.
Powell added: "I don't think it's a red and I don't think there should be anything else come from it – the sending-off was pretty harsh really so I'm hoping he'll be OK (from suspension).
"You start watching the clock because we'd started the second half so well and built up a commanding lead, then you go down to 12 against a team that is chasing the game. I don't think it was the right call, but we had to deal with it and we did that pretty well."
Leigh head coach Adrian Lam blamed a "diabolical" start to the second half for his side's failure to wrest their way into a second consecutive semi-final.
Armstrong's try had hauled his side back into the contest, but the quick tries early in the second period gifted the hosts the commanding advantage that they did not let slip.
Lam told Sky Sports: "A couple of soft tries was exactly what we didn't need – the story of the night. We rallied, but the first five or seven minutes of the second half was diabolical.
"It was quite a frustrating watch at times. There were too many mistakes and everything we tried not to do.
"Everything we've not done this year has all come out tonight, for some reason. It's down to execution and individuals in big moments."
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