This is a drill to work on your release, flexing, extending and the retraction release - focusing specifically on collapsing the outside strong leg, crossing over flexed legs and extending on the other side, while keeping the hips low and level.
This drill allows a lot of power to develop on the outside ski - don't rush to this drill before mastering tipping the skis on edge smoothly - by its nature (i.e. wide stance, outside ski focus, power), this drill can interfere with tipping somewhat.
There's not much to say - just go out and do it, take video and get feedback! We'll underline the main points of focus below.
Warmup well and get into a really wide stance, on a blue or black run and start carving it. Focus on getting a long and strong outside leg at the apex.
Ride the outside ski with that long and strong outside leg. grinding the ski around the turn and, as it points across the slope, start flexing and collapsing the leg, while keeping the hips level as much as possible: avoid any pushing "up".
Keep collapsing the outside leg and flexing it and, as the hips cross the skis, keep pulling the now inside boot up, until the skis get separated quite a lot, the new outside leg is extending (without pushing) and your hips get very low and the new outside ski is long again.
When extending the new outside leg, make sure the extension is passive, i.e. you're not pushing the leg to an edge.
It is specific to this drill to open up your stance at the apex a lot as you explore the extreme of pulling the inside boot "up" and flexing the inside leg to create a very long and strong outside ski...
Watch the video: youtu.be/FB8Nb8FTeoU
This is a drill you should use often. Do not push on that strong outside ski to get off it, just focus on flexing the leg and untipping to end the turn. Also, don't push into the new outside leg to move the hips into the turn, but rather focus on pulling the inside boot up.
Here's another perspective:
Because during this drill, you focus on pulling the inside boot up, the inside ski is less weighted and it helps create big angles. You can use it as a carving drill, speed management on the steeps etc.
The instruction was to "keep the hips level as much as possible" and avoid pushing the hips up at all". In reality, the hips will come up slightly, if you manage big angles. Big angles require the hips to be lower than the knees and safety requires the hips to not be below the knees.
So, as you cross at skis flat, the hips should be no lower than the knees, roughly, otherwise you endanger the ACLs, especially if you get attacked by a snow snake. However, flexing that much is already a huge range of motion for most skiers - a large majority of which lack flexing completely.
Here's another view: