This is the sort of thing that you need to start ignoring if you are going to learn any QM.Here is another study that explores the same question:
"REHOVOT, Israel, February 26, 1998--One of the most bizarre premises of quantum theory, which has long fascinated philosophers and physicists alike, states that by the very act of watching, the observer affects the observed reality. "
"the very act of watching, the observer affects the observed reality"
How does the "observer effect" cause the two different output patterns in the double slit experiment for electrons? There is a different output pattern when the electrons are being observed before the going through the slits and when they are not being observed before going through the slits.
There is a common experiment where you measure the speed of a bullet by firing it into block of wood and measure the momentum of the block after the collision. In this case, the measurement of the bullet's speed/momentum fairly brutally changes the speed of the bullet. This is clearly of no significance to the question here.
The fundamental point about the double-slit experiment is that if you do not measure which slit the electron passes through, then the question of which slit it passes through no longer has meaning.
In the Feynman lecture (which you really should watch), he sums it up as follows (in the case where thjere is no detector):
Proposition: the electron must either pass through slit A or pass through slit B.
He then shows that, using the experimental results of the double slit, that proposition fails. That proposition seems so close to basic logic that it is hard to understand how it could fail. But, the proposition fails, nevertheless. You cannot say that the electron passed through either slit A or slit B; and, you cannot say that the electron passed through both slits; and, you cannot say that the electron passed through neither slit. You cannot say the electron behaved like a particle; you cannot say the electron behaved like a wave; and you certainly cannot say that an electron behaved like a wave until it was observed and thereafter it behaved like a particle.
This is where it becomes fundamentally important to understand the electron not as a classical particle that has a well-defined trajectory, but as a quantum object whose position (if and when you measure its position) is governed by a probabilistic wave-function.
The electron never is anywhere or doing anything that is changed by an observation. That's the difference with the bullet and the block. The bullet, statistically at least, really has a well-defined position, momentum and trajectory which is changed by its collision with the block. It makes no sense to ask where the electron was if you didn't measure where it was.
One final point is that it is also worth studying the single-slit behaviour of quantum objects. Another lecture I would recommend in this respect is the following. The single-0slit experiment is analysed and carried out from about 32 mins in (although the whole lecture is worth watching if you want to understand QM).