The way I think about it: Pikachu can die from ftilt-up smash true combo anywhere from 78-91%. Sheik's needles go through Pika's thunder jolt, which will not kill me even if I'm at high damage. So as soon as I see a thunder jolt, Pikachu is at 18%. If I land an ftilt on Pikachu, I can probably do another one and finish it with up tilt, so 22%. If I land another ftilt, I can probably do it again and finish off with nair, ~20%. Ftilt, ftilt, nair ~18%. Pika's at minimum kill percent and I can either go for the kill and hope he doesn't DI or just do something else to ensure death. If I fail, transform to Zelda. If I can avoid being killed, it's all I have to do. If I have stock advantage, I can pick and choose my openings. If I die, back to Zelda, hopefully having done at least enough damage for a dtilt lock.