The aim is time above 90% of VO2 max, and the trick is that the off is not off. Thirty seconds at 125–130% of CP drives oxygen uptake up fast; the sixty seconds that follow sit at 100–102% of CP, just over the line, so uptake never falls back before the next surge. Four five-minute blocks stack twenty minutes of that state into an hour.

Compared with a classic 4x4, the strain per minute is lower and the time near maximum is higher, because the body is never allowed to settle. The two-and-a-half-minute recoveries at 40–50% are short on purpose: enough to repeat the block, not enough to reset it. The four-minute ramp from 80 to 100% in the warm-up gets the system moving before block one.

In Vekta the surge and the hold are written as shares of CP, so the hold really is at the rider's own CP and not a guessed FTP. The W′ preview shows the battery dipping on every surge and only partly refilling on the hold, which is exactly the shape you want; the interval analysis afterwards shows how much of the twenty minutes was spent above 90%.

A VO2 block session, one or two a week for three to four weeks, indoors or out. If the sixty-second holds drop under CP in block three or four, the surges were too hard.