The defending champions had laboured to break down the Portuguese defence, but the second half introduction of Cesc Fabregas, Jesus Navas and Pedro Rodriguez gave Spain extra vim.
There were no goals in the 120 minutes at Donbass Arena on Wednesday, but Spain held their nerve in the ensuing shoot-out, with Fabregas netting the decisive penalty to send La Roja into Sunday's final in Kyiv.
Del Bosque's substitutions were a key feature of Spain's 2010 FIFA World Cup™ success in South Africa two years ago, with Fabregas and Navas both coming on to good effect in the 1-0 victory over the Netherlands in the final.
Against Portugal, the three replacements gave Spain greater dynamism in the final third and Del Bosque drew attention to their contribution. "Neither team had many chances to score. We had to improve this and I think that after Fabregas, Navas and Pedro went onto the pitch, we caused them more problems," he said.
"It was vital. The players are getting more mature with each match. Extra time showed that the players have gained experience of how to manage matches. [Portugal] were superior in defence. We didn't have that many opportunities so it was very balanced. I think in extra time it was a bit different. And then we had lots of luck in the penalties.
"I want to congratulate Portugal, because they played a great tournament, but we were luckier this time."
Cristiano Ronaldo had been identified as Portugal's dangerman before the match and despite a few scares, most notably when he sent a shot over the bar in the last minute of normal time, Spain managed to keep him at arm's length.
The Portugal captain was kept in check by his Real Madrid colleague Alvaro Arbeloa in the Spanish right-back position, but Del Bosque said the entire defence deserved praise for their efforts. "We don't just want to defend," said the Spain coach. "We want to attack, too. But of course we are improving our defence and that's thanks to the qualities of our players.
"Stopping Ronaldo was mainly Alvaro's task but everyone helped out - Gerard Pique, Sergio Ramos, Jordi Alba. They all had an excellent match. The defence helped a lot and Arbeloa played well, as he did during the whole year."