Oscillocope interleave mode and repetitive mode
I'm slightly guessing here, and I think you are using an analog scope. The average oscilloscope is single beam and can therefore only draw a single line at a time. But you have two (ore more) input channels. To be able to draw both channels two different tricks can be used, mainly depending on the selected time base. The commonly know terms are as follows:


'alternating' for 'repetitive' mode.
'chopping' for 'interleave 'and 


Alternating 

The simplest mode to show two or more traces on an oscilloscope is alternating mode. A full timebase sweep is made by drawing the first channel full left to full right on the screen, then the timebase is restarted and draws the second channel again full left to full right. This repeats itself indefinitely and when fast enough, it appears to the eye asif both traces are drawn at the same time.



The disadvantage of alternating mode is that it takes at least twice as long to refresh a single trace and the display will start to flicker nervously when the refreshes for a full screen drop below approximately 20ms.

Chopping

Some oscilloscopes will automatically switch between chopping and alternating mode, some have a manual switch. Rule of thumb is that for time base setting at 1ms/div or below you want to switch to chopping mode.

In chopping mode the oscilloscope switches very fast between the channel inputs, drawing an extremely short subsection of each trace. As the scope manages the beam to switch very fast between the two traces, the vertical line will not show, but input traces will. 

The disadvantage for chopping mode is that when the time base setting gets lower, the screen updates faster, the chopping effect will start to show on screen.

Digital

Digital scopes, and especially the ones with an LCD display, don't suffer from this problem as these displays don't use a single electron beam to draw the display but an LCD panel instead. You won't find this setting on digital scopes.

Found the image in this article here.