Armstrong scored a wonder goal in last week's two-legged, quarter-final win against Guildford Flames and, while his effort on Saturday evening to seal a come-from-behind 5-4 win over arch-rivals Nottingham Panthers didn't quite match that, its importance more than did, his solo effort sending the hundreds of Steelers' fans into raptures.
It meant heartache for Panthers' exiting head coach Corey Neilson, who is bowing out after 10 years without adding to the three play-off titles he has already won for the East Midlands club - among many other honours.
Paul Thompson's team went two goals behind twice and showed tremendous character to come back, particularly as the equalising goal from Levi Nelson came with only 99 seconds left on the clock in regulation.
It means they have an opportunity to replicate their heroics from 12 months ago when they edged out league champions Cardiff 6-5 in the second period of overtime.
Thompson surprised some with his team selection ahead of the game, a fully-fit roster meaning he had to sit two imports. He opted to recall forwards Tim Wallace and Matt Marquardt, who had been sat out against Guldford, in place of Swedish forward Andreas Jamtin and Finnish defenceman Miika Franssila
And there will have been some seriously questioning the Steelers' coach's decision after his team got off to the worst possible start, however - Alexander Mokshantsev dancing around the back of Ervins Mustukovs's net to back-hand the puck over the shoulder of the netminder from a tight angle with just 47 seconds on the clock.
After that initial shock, the Steelers settled and almost levelled when, through a 3-on-1 when Colton Fretter rattled Patrick Galbraith's left-hand post.
But another soft goal had Thompson's players truly up against it, a pass down the middle of the ice from former Steelers' forward Robert Farmer found an unmarked Luke Pither, who danced clear through on Mustukovs before firing the puck over his right shoulder and into the net at 12.25.
The Steelers halved the deficit thanks to some quick-thinking by Mathieu Roy, who back-handed a mid-air puck over the line before Galbraith could get his glove to it. It required a video review from the officials to confirm it had indeed crossed the line, but they were back in the game.
They then drew level just under five minutes into the second period when Jon Phillips was first to react to a rebound in front of the Panthers net.
The lead lasted less than two minutes. however, when, with Joonas Ronnberg in the box on a holding call, the puck was worked to Pither on the right-wing hash marks from where he rifled in over Mustukovs's left-shoulder and into the top corner.
The Steelers had just killed off another penalty - Zack Fitzgerald sent to the box for interference - when the Panthers established a two-goal lead for the second time in the match, Mustukovs' initial left-pad save from Robert Lachowicz going straight into the path of the oncoming Jeff Brown who made no mistake for a 4-2 lead at 33.45.
The third period seemed low-key compared to the first 40 minutes, but it became a one-goal game again in the 49th minute when, on the powerplay, the Steelers were patient and worked an opening for defenceman Ben O'Connor to fire a rocket into Galbraith's top left-hand corner.
As the Steelers' pushed for an equaliser it naturally left gaps at the back, with the Panthers left to rue a miss by Raphael Bussieres with an open net beckoning.
How much they were to regret that miss - they also went close again in a goalmouth scramble shortly after - became clear as the game entered its last couple of minutes.
Robert Dowd gathered the puck on the right-hand side and fed a precision pass into the path of a charging Levi Nelson - scorer of last year's play-off final winner - who deflected it through Galbraith's legs. Although the puck edged over the line, the officials went to a video review again to confirm it was a good goal.
It set up 10 minutes of fast-paced 3-on-3, with the Steelers enjoying the better puck possession. Fretter went close after dancing through down the left before Armstrong weaved his magic - something the Steelers have misssed for almost half of the seaon due to a hand injury - closing down on Galbraith's goal before flicking the puck over his shoulder and into the net.