Block voicings (also called locked hands) take a 4-way close voicing in the right hand and double the top note an octave below in the left hand. The melody rings out on top AND bottom with harmony sandwiched in between. The result is a thick, punchy, horn-section-like sound.
RH: close position chord with melody on top. LH: melody note doubled one octave below. Total: 5 notes. Both hands move in parallel — 'locked' together.
For melody statements where you want maximum impact. The George Shearing quintet sound. Red Garland used this technique extensively. Works best at medium to up-tempo — at slow tempos the density can feel heavy. Requires solid 4-way close knowledge first.