When people first learn the Von Flue choke they often find that their opponent can bridge them over and then they end up on the bottom.
If this is happening to you and you’re getting rolled into a bad position when trying this choke then first check that your legs are perpendicular (i.e. at 90 degrees) to your opponent. If his head is pointing to 12 o’clock, then your legs should either be to 3 or 9 o’clock. The closer your legs get to his head the easier it is for him to bridge you over.
Also keep in mind that some grapplers just have very explosive bridging power. If I was unsure about my base and going against one of these bridging dynamos I might choose NOT switch my legs, and simply use shoulder pressure. It’s the same choke, just relying less on body weight, more on arm strength, and a bit slower than the inverted-leg-switching Von Flue choke I showed in aforementioned blog entry.
To illustrate this non-leg-switching variation, here is another video of another Von Flue choke in action, starting at about 1:00 in the video (thanks to TravenBJJ on the underground forum for bringing it to my attention):