David's Tip of the Day: Chamber Size and High Bends, Part 3 - Tips from the Harmonica Masters Workshops

In regards to the 10+ bend... If your tongue moves forward in your mouth, the 10+ bend will start in the fully-bent position (10"+... 10 blow whole step bend)... in this case the Bb. This is due to the fact that your mouth is starting from a low tuning, and as your tongue moves forward the tuning of your chamber raises in pitch until you reach the pitch of the 10"+, and the bend sounds. In order to hear a slide from 10+ (C) to 10'+ (B) and ultimately 10"+ (Bb), you need to start with your tongue forward, tuning your mouth HIGHER in pitch that the bend.

So, the key element for creating a slide from the 10+ down to the 10"+ is in starting with small chamber, and then just as in standard draw bending you move your tongue back. This is also the case for the 9'+ and 8'+, so make it a rule of thumb that when sliding from a natural pitch of a reed downward, your tongue (or the constriction point in a wave-like motion) always moves back.

P.S., the 8'+ and 9'+ can be bent with a good slide even when moving forward, but it's best that you approach all of your bends with this front-to-back motion, your bends will be more consistent for it.