You may think of the A intensified by B display mode as having to do with separate time bases and alternate sweeps, but it doesn't.
The first thing you may notice is that the time bases are arranged in a 1-2-4 sequence rather than the traditional 1-2-5 sequence.
For time bases slower than 20 ns/div in the FastAcq mode, ETS is deselected.
5-MB memory length is used, at some time bases, it will reduce to 20 Mb or 2.
For example, if 5 GS/s were chosen, as slower time bases are selected, more and more memory will be assigned to a channel.
As Figure 3 shows, Tektronix adopted a 1-2-4 time-base sequence with small changes to mostly binary memory lengths at slower time bases.
As shown in Figure 3, at 2 ns/div and faster normal-mode time-base speeds, and for the same memory length, the time base in the normal mode is 200x faster than that indicated in the FastAcq mode.
In the Agilent Help section, the bandwidths are listed that correspond to each combination of memory length and time base.
For example, if you have selected an 11-s pretrigger delay, single-channel operation, and a 200-[micro]s/div time base, the sampling rate will be 500 kS/s for any memory length greater than 2 kwords.