In the video and the stills there are big vents seen on the top of the unit. On the bottom backside there could be two much smaller intake vents  spotted. The underside of Switch is almost never seen - only the last pic on nintendo.com shows something gray/white on the left underside.

So maybe there is active cooling going on or by convection when mobile.

If only convection is used that might also open the possibility that the docking station has a fan and channels fresh air throught the backside of the Switch so that much more CPU/GPU power would be available when used stationary.

Having a fan within the main unit would mean that this fan must be very small, silent, dust resistant and running in a "non disturbing" frequency - something which is very hard to get. Oh - and a fan would consume power too which is something to avoid while being mobile...

Also nothing was shown from the backside of the dock which does put more oil into this speculation... .

The only reason for those top holes / vents would be some form of active cooling. Nintendo is not putting those in for esthetics :p.

I would estimate that the upper outtake vents are at least four time the size of the intake vents I've spotted on he backside. Makes no sense to me as this would mean that an internal fan would have to overcome that "artificial" air flow barrier and such wasting electric power.

On the other side mobile cooling by convection would mean that the outtake has less pressure (which is anyway the case with hot air)...

It would also be possible to have an second fan in the docking station if the one in the Switch main unit could be "put off" from the electric connection so it won't act as an electric generator...

I hope Nintendo has a great solution as I've bad experiences with little fans and conjunction with dust/smoke. Even my Wii U fan died and had to be exchanged (under warranty) and I know at least one other person which had the same problem...

Can you provide a picture of those vents on the back? Or a timecode on the trailer.

I think i saw two ones at the sides below, but i'm not sure if that could work since that side would at the bottom when standing up.

I'm kinda heartened by the display of vents there. It means Nintendo went with a fully featured Tegra X1/X2 from the looks of it, not some gimped version of it for the sake of passive cooling.

I can live with a fan if it means better graphics performance. My laptop has a fan, I don't care. The Surface tablets (the higher end ones) have a active fan too.

Is it confirmed to be for cooling? I mean Nintendo consoles are known for having stuff on them that never get utilized. I mean it could be for sliding the tablet into a stand for the car like seen in the video.

If've seen them only on the gallery too (left2right, top2buttom): pictures 4 and 9. And those backside vents look large but are covered. So the air can only go trought the sides of that cover (pic 9)... . IF these are vents at all and not something "special" to secure the while being in the dock... . 

Another possibility is that the upper vents are for intake and outtake as pic 4 shows a horizontal plastic but it remains unclear how depth this is...

The upper vents: for sure. you can even spot a screw on the right side of the unit (pic 4 in nintendo.com gallery). I guess if there is a fan it is part of that construction...

But still the question remains: the air they blow out, where does it come from?

iPhones have passive cooling too (while getting a bit hot) and nobody is whining about too less power (besides the battery).

So if Switch has an updated Tegra which might consume far less power and runs much cooler when being mobile those holes would do wonders if combined with a tiny chip cooler.

The dock is such big they could put in fans which provide "more than you need" cooling when stationary. We have to remind that the "cool" intake air can get hot during summer...

You are supposed to cool it yourself with water.