Phoenix Hunter: the cross ankle release "at the pole" (dumb name, I know) is set up just like the Dangerous Brian, which Veena kindly just posted for visual reference...but it is NOT a thigh hold.
Imagine the Dangerous Brian but with the "free" or back leg also brought to the pole so that you can cross that ankle behind the hooked leg and squeeze the pole between the meaty sweet spot inside both knees. It is super secure.
For entry, you go from a pole sit with legs extended, basically a pike, like you're in a plank but sitting up with the torso vertical, with your hands below chest level and lean torso back so your legs come up a bit. From there I have seen two common methods of leg placement. One is to keep the legs crossed at the ankle and slightly bend the legs outward which creates an opening for the pole between your knees and then pulling your legs in toward your body until the ankles meet the pole and the pole is between your knees so you can grip it, then of course laying back. I simply cannot do this entry for some reason. Plus I don't like the knees bending, which loses the pretty line. The other method is the same in the beginning, but then instead of leaving legs crossed, you lean back with your hands still gripping right in front of you, uncross, and bring your hooking leg only to the pole and hook it. My other leg, which also gets to stay straight and pretty, then comes to the pole, finds the perfect knee placement on the side, and then the ankle meets the other ankle, crossing behind. This may require some finagling with your hand placement, but you'll find something that works for you with practice.
At first, it can be challenging to find the sweet spot between the knees for many people, but the correct knee placement is more important than having the ankles crossed just right, because it's really the between-the-knee grip holding you. I find that when I find that perfect side knee squeeze grip spot on the second leg, it's tough to bring the second ankle to completely meet the first. It does, but if I'm focused on crossing harder at the ankles from the get-go, I end up with a nasty bruise from bad knee placement.
Once you have mastered this one, you can do the one-legged variation (which is still not a Dangerous Brian). Everything is the same only the free/non-hooked leg, once brought to the pole and finding the good spot on the side of the knee and squeezing the pole between both knees, BENDS and points toward the floor. It looks much more impressive but really isn't much harder, as the ankle cross isn't doing much to hold you up. It just feels more secure.
Like polekat said, using these to pull up from a headstand/elbow stand/handstand is the best and safest way to learn.
As Veena said, the Dangerous Brian is definitely more advanced.