sideline and defensive position

[so cal lurker]

I'm helping coach a jr high team this year. IIRC there was a change a few years back about defensive position and the sideline . . . but I'm not sure about my memory on it or how it actually called.

When a defender chases a dribbler to the sideline, the way I was taught (more than a few years ago . . .) was to plant a foot on the line to prevent getting beat that way. I have a vague recollection of a change that would -- at least technically -- make that not legal defensive position and preclude drawing a charge.

So I have two questions:
(1) is that in fact the technical rule? (Or am I misremembering?)
(2) is that how it is actually called?