Here is an interesting refereeing situation about allowing direct access to hit the ball onto the front wall. Shelley (in blue) must give direct access to the front wall for Nicol's shot. At first glance it looks as if Nicol, should have a stroke, as Shelley has not done this.
However, the referee has ruled this situation as a LET. This is because in their judgement as the ball is so close to the backwall Nicol could not have hit the ball cross-court, in which case Shelley is not in the way. Nicol may have been able to get the ball straight, which was potentially dangerous so therefore a LET was awarded.
So in this situation Shelley is causing little or no interference to Nicol, but Nicol has stopped in case she might hit Shelley with the ball. If this ball came further off the back wall, and Nicol would have had a full choice of shots, then a stroke to Nicol would be the correct decision, as the path to the right hand side of the front wall is very much blocked.
It's worth noting that the urban myth of "no strokes given off the back wall" is completely incorrect. There is no reason why a stroke can't be awarded after the ball has hit the back wall.