The Flash and Supergirl stare at each other in new crossover teaser
The 10-second teaser spends half of its (literal) running time showing Flash zipping along the Supergirl crest before he appears in an alley. Supergirl then lands across from him, and they stare.
Don't worry, there'll be plenty more action to come when the episode premieres on CBS on March 28: according to the network, Flash appears in Supergirl's universe and teams up with her to fight Livewire and Silver Banshee. In return, she helps him get back to Universe CW.