Brush and spank are sounds made by the ball of the foot brushing across the floor. Learn them lightly and in time: foot motion, timing, mistakes and milestones.
The honest answer first: brush and spank are sounds in tap dance in which the ball of the foot sweeps forward (brush) or backward (spank) across the floor. Unlike the drop, the foot does not fall, it glides. That gives a short, soft tone that appears as a building block in many steps.
The brush sounds unremarkable but is the basic building block of shuffle, flap and time step. Anyone who plays brush and spank cleanly has laid the foundation for almost everything that follows. And because the foot needs little force here, you can practise the sound quietly and in control.
Research shows why the foot is so busy: in a 3D motion analysis of three experienced tap dancers, feet and knees were the main actors; the hips moved no more than 21 degrees in two of the three analysed steps, whereas the ankle moved at least 10 degrees per step [1]. So the sound is created below, not above. A "brush-brush-stamp" took the dancers 1.36 seconds on average [1], about as long as it takes you to count to "one-two".
The roles of the sounds matter: the brush is the soft lead-in, the spank the clear follow-up. Together they make what tap dance calls a "shuffle", and so belong to the first things you hear in any class (common practice standard, no source).
Realistic: brush and spank sound roughly right after one practice evening, clean and even after a few weeks (rule of thumb, not a study result).
Play ten heel drops and ten toe drops in time. Milestone: all sounds sound the same, no wobble in volume.
Lift the foot slightly. The ball sweeps forward across the floor and produces a short tone. The knee stays soft, the movement comes from ankle and knee, not from the hip. Milestone: ten brushes of the same length and sound.
The spank is the sweeping movement backward. The ball touches the floor briefly on the way back. The sound is usually shorter and a little harder. Milestone: ten spanks with an even sound. You hear the difference from the brush.
Play brush and spank alternately, two sounds per beat. That is the basis of shuffle and flap. Start slowly, around 60 beats per minute, one per second. Experienced dancers detect small timing deviations in tap dance sequences better, especially when hearing rather than seeing them [2]. Use that: trust your ear, not the mirror. Milestone: eight repetitions in time.
Play brush and spank with a metronome without rushing. The foot swings, it does not push. Milestone: twenty clean repetitions in a row.
Brush and spank load the ankle mainly through repetition. In a study with 14 tap dancers, steps such as cramp roll and toe cannon produced clearly higher ground reaction forces than others [3]; brush is comparatively gentle, but with many repetitions you tire. Warm up ankles and calves for five minutes, build up tempo and duration slowly and pause if your shin or foot hurts. This guide explains and motivates; it does not replace supervised training.
Film your foot in slow motion from the side: you see immediately whether the ball glides across the floor or strikes, and how high your knee really goes. Ten seconds of video often replace ten minutes of guessing. Next up is the ball change.
The brush is a sound in which the ball of the foot sweeps forward across the floor. It is short and soft and forms a building block of many further steps such as shuffle and flap.
The spank is the sweeping movement backward: the ball touches the floor briefly on the way back. The sound is usually shorter and a little harder than in the brush. Brush and spank are often played together.
Slightly. In the drop the foot falls, in the brush it glides in a controlled way. That needs more feeling for height and tempo. With a secure drop, the brush usually sits after a few practice evenings (rule of thumb).
Not fast at all. Cleanliness first, then tempo. Start at one sound per second and raise it only when every sound is the same.
Usually you press the ball too hard onto the floor or brush too slowly. The brush needs a short, light contact and a quick glide out. Lift the foot only a few millimetres and keep the ankle loose. Also check that the taps are tight.
Ball Change in Tap Dance: The Quick Weight Shift From Ball to Ball