Wait Till 8 at 138 is a drill to practice the timing of large taiko strikes. Ideally, acceleration begins from the apex of the strike and continues throughout the downstroke. A common problem, however, is "easing" into the strike, with acceleration only starting midway through the downstroke. This is a result of "putting the brakes on", in order to keep the hit from landing early. To fix this tendency, we need to understand exactly how long the full strike should take, and then when the acceleration should begin in order to hit at a specific point in time.
Gravity as base-line
We start by calibrating our strike to the speed of falling with gravity, using a metronome set to 138bpm. At approximately 4 feet above the taiko, the bachi takes approximately .5 seconds to free-fall to the drum face (see calculation below). Because the height of the bachi tip is often a bit higher than 4', and to make sure I'm not "putting the brakes on" during the strike, I use 138bpm, which works out to .43s between clicks.
Step 1 - Gravity: 138bpm
With the metronome set to 138bpm, practice striking with the R hand every 8th click. "don 2 3 4 5 6 7 8". The arm should wait until the 8th click before letting the arm fall. A perfect free-fall with gravity should should hit precisely on the next click (the "1"). Practice with the L hand. Then alternate hands: "don(R) 2 3 4 gon(L) 2 3 4".
Gravity with a boost
Step 2 - Additional acceleration: 160bpm
Same practice but at 160bpm. These faster strikes should still accelerate smoothly, as if the arm and bachi are still free-falling, but due to slightly stronger gravity.
Step 3 - Additional acceleration: 200bpm
Same practice, being careful not to feel unduly rushed.
Starting down-stroke without a click
Step 4 - Breaking strict relationship to click: 69bpm
Set the metronome to 69bpm, 1/2 of the "gravity" speed of 138bpm. "don 2 3 4". Do the same practice as before but now the arm will begin falling on the "&" of 4. The timing of the strike should be identical to the 138 practice, only now the moment of starting the strike has to be inferred (no click exists at the moment the arm should start falling).
Step 5 - Falling on "e": 104bpm
The next step practices maintaining this "gravity" acceleration with a metronome that doesn't quite match the fall timing. At 104bpm, the arm should begin falling on approximately the "e" of the 4th count. Practice as before: R, L, alternating.
Musical application
Step 6 - Application to M1-4
Practice M1, M2, etc at 138bpm, making sure the large strikes within each rhythm begin their falls at the proper times (with a click).
Slow tempo
Step 7 - Slow metronome, fast strikes: <50bpm
At this point hopefully we have internalized what the "gravity" strike timing feels like. We now need to be able to maintain this strike speed when playing to a very slow metronome. Set the click to 50bpm and practice M1, M2, etc, making sure the strikes don't "ease down". The extra time available (compared to 138bpm) should be taken up in a slower upstroke and/or waiting longer at the top. The down-strike should take the same amount of time as before (but will have an ambiguous relationship to the metronome.)
Calculating time to fall 4'
t = sqrt{2 * h feet / 32}
t = sqrt{2 * 4 feet / 32}
t = sqrt{.25}
t = .5s