you can get behind them by going through them. i have practiced with folks whose training involved staying on the line and taking the center as well as folks who always getting off line. i found that folks who trained to stay on the line didn't have problem getting off line; that's just another option for them. however, folks who always trained with getting off the line, had a hard time stay on the line when required, i.e. put them on a log bridge exercise.
Great comment. "Once off line, Uke have more time to continue/mount another attack. Please consider staying on line and use yin and yang. Demonstrated by videos of Yamashima sensei, Bill Gleason sensei and many others who uses ground path and yin and yang or Bruce Lee's movies. One of Bruce Lee's favorite is to "dissolve the attach and attach (enter) at he same time". Again, yin and yang (O-sensei's secret).