Confusion in hour glass time measurement problem

#1

Hello,
I watched this hour glasses time problem video, but am getting confused on the what the main idea is. There are 2 glasses : One measuring 11 minutes and another measuring 7 minutes. One the second solution given in the video (to exactly measure 15 minutes), I am getting confused.

Does that mean that the first glass would be let run for 11 minutes at first, and once that glass has filled, then exactly at the time when first glass is completely filled, second glass is used to measure the 4 remaining minutes. However, how do we time that 4 more minutes since we don’t have any timer. The video says first to measure 3 minutes, then 4 minutes are remaining. However, I am getting confused on how the 3 minutes are measured with this 7 minutes glass. Could you please clarify?

Thanks,
Joyjit

#2

Sir mentions the states of jar. Let me start from beginning.
You start both the jars.
After 7 minutes, the 7 minute jar gets empty, you flip it and it again starts emptying.
Now at 11 minutes , you know the 7 minute jar would end in 3 minutes. But if you flip it, it will empty in 4 minutes, thus after 11 mins you flip it, and no when it gets empty we know that it’s 4 minutes more.

Thus 11+4