Bud Powell, Barry Harris and other bebop pianists used this left-hand, self accompaniment technique almost exclusively, in both solo piano and group contexts. Employing shells in the left hand is an easy and effective way to outline a song's harmony.
Shells consist of the root (R) plus the 3rd or 7th of a chord, in the following configurations:
R + 7 (or 6)
R + 3
R + 10
R + 7/6 + 10
Within a ii-V-I harmonic progression, shells would be played as follows.
The large intervals make playing in the lower register possible, without sounding too muddy. With only 3 notes; 2 in the left hand, and a single note melodic line in the right hand, a solo piano "arrangement" can sound surprisingly complete.
Large, 3-note shells:
When using shells, left hand rhythmic activity must be kept to a minimum. Shells are played primarily on downbeats and beat 3, if there are two chords per measure, or on their anticipations (+ of 4, + of 2).
On several occasions I have heard jazz educators instruct young pianists never to play the root in the left hand, as "it will get in the bass player's way". Perhaps these teachers do not own or listen to any Bud Powell recordings, because Bud almost always player left hand shells with roots. Nothing clashes with the bass because both the pianist and bassist strike the root at the same time. Granted, in more contemporary jazz contexts, more harmonic freedom and interplay is achieved and enjoyed through the avoidance of LH roots and instead, employing rootless voicings.
Homework Practice Assignments:
1) Play through the attached "Shells Excercises" page. Observe how the shells are constructed. See the patterns: each ii-V-I progression descends by a tone, and consists of the same numeric formulas. Strive to quickly comprehend the gist of the page, committing it to memory. Repeated readings is not the intent. Play it through with a metronome clicking on beats 2 and 4 at various tempos.
2) Three different approaches to playing left-hand shells on the jazz standard, "Just Friends" are attached. Play through each, observing how the shells were constructed.
3) Using the "Just Friends - Small Shells" attachment, try playing the melody of "Just Friends" in the right hand, with small shells in the left hand.
4) Work your way through a fake book (the Real Book, New Real Book, Colorado Cookbook, etc.), playing the melody in the right hand and shells in the left hand. At first, do not mix large and small shells. Decide which size you will use, and stick with it for the entirety of the song. Once you have mastered each variety, feel free to mix and match.
The more you try applying this technique, the better you will become at using it in practical contexts.