This exercise is a lot like the Add a Note technique, but it's specifically for speeding up sections. There are two options: forwards chaining or backwards chaining.

Forwards Chaining:
1. Choose your unit; you should already be able to play it perfectly at half tempo.
2. Play the first two notes together as fast as you can 5x. You must be disciplined not to go on to the next note.
3. Add the third note and play all three together as fast as you can 5x.
4. Continue in this same manner. 

Backwards Chaining:
1. Choose your unit; you should already be able to play it perfectly at half tempo.
2. Play the last two notes together as fast as you can 5x. Pay attention to bowings!
3. Add the previous note and play all three together as fast as you can 5x.
4. Continue in this same manner. 

For both of these chaining techniques, you'll find that adding a note can cause the previously chained notes to break down a bit. This is normal, and you can usually get them back up in the 5 repetitions. If not, add a few more reps.

If there's a moment in the unit where you consistently break down, isolate that moment and the 2 or 3 notes involved. Begin a new chaining exercise around those couple of notes.

EXAMPLE

Bela Bartok: Concerto for Viola and Orchestra, 1943
First movement: Moderato

Paul Hindemith: Sonata for Viola and Piano Op. 11/4
First movement: Ruhig