Leaving Out the Bass Register

A very effective way to create colourful orchestration and dramatic variety in a piece is to not have the bass registers present all the time. A conscious decision to free up the lower register will create a very dramatic effect once the bass register enters back in.

The passage that is left without bass will ideally create an elevated quality, somehow feeling "suspended in the air" and regaining gravity once the basses enter again,

A classical way to apply this would be to leave out the basses in the B section of a theme to bring it back in when the theme hits again. A little less popular but nevertheless brilliant example for this concept would be the "Buckbeak's Flight" Sequence from Harry Potter and the Prisoner of Azkaban,

Observe how the first part of the theme covers the full range of the orchestral forces with Double Basses providing the fundament. With the entrance of the B Section at 0:50 John Williams drops the entire bass register which supports the "flying" quality of this sequence. During the development of that passage, he gradually brings the basses back, first rather sparsely and with "drop outs" again, additional pizzicati and only relatively close to the lift into the climax at 1:21 we have a full fledged bass register again.

You can observe the same approach in quite a lot of compositions by Williams but also other composers.

The most noticeable way to achieve this would be to leave out the Double Basses as they create a lot of carrying power as soon as they play but most effective is this when you empty out the entire bass register and leave out Tuba/Trombones, Bassoons/Contrabassoon and low Celli (registers). 

By the way this concept works with quite a lot of factors in music. Losing something for a while that seems like it should generally be there will create a strong force as soon as it returns. This works for example also on a rhythmical basis. Long chorale notes entering after an extensive passage of quick notes as for example in Britten's Fugue from The Young Person's Guide to the Orchestra is a great example for this.