Ardha Vinyasa. Half flow. It's that transition through chaturanga to upward facing dog to downward facing dog; and you'll do it multiple times in a vinyasa flow class.
It requires a certain amount of core and arm strength to move through it properly, but it can be tricky when you are first getting started or even if you've been practicing yoga for a while.
So how do you modify and build strength? Here are three progressive options:
1) Instead of going through the flow, hold your plank and focus on building core strength.
2) Plank > lower all the way down> Cobra pose> Push up with knees on the ground for support. (Focus here on the eccentric contraction in the triceps: lower down as slow as you can).
3) Supported chaturanga hold & push back up. Check out the video below to see how it's done.