The Downthrust is performed by pressing down while jumping.

How do you attack while gliding?

Like Climbing and Swimming, the player cannot perform combat actions while gliding, with exceptions. To initiate gliding, press the 'jump' button mid-air to open your Wind Glider. To cancel gliding, press the 'jump' button again or initiate a plunged attack to slam down to the ground.

How do you attack with glide on Botw?

Press the attack button while gliding. It's a little silly, but there's a different aerial attack for each type of weapon, depending on whether you were using the glider or not.