He knew he stood no chance against Cell in either form at that point. It was a risk for him to pop in the way he did and save them. Luckily, Cell was perplexed by his arrival and didn’t see him as a threat.I watched the fight between the Tien and Cell but there was something that bothered me when I was wathcing it. Why didn't Goku rescue Piccolo and Tien when they were getting beaten by Cell?
Yeah, Goku wouldn't have been very effective at that point. Also, it would have taken away from Tien's awesome moment when he tri-beamed the hell outta Cell to save #18 and prevent Cell from consuming her.