Gaining

No it's just hot

That's it really. The funnel is just for the fun of seeing how much you can get down quickly. No time to feel full - you are just limited by stomach capacity.
I do it with beer but gainer shake can be used. You need a tap on the tube to control things and something to hang the funnel on. I find I can take a maximum of 3 pints at a go but start less.

Its the loss of control and knowing everything is optimized for fat consumption that makes it hot. You have no choice when funnel fed, you're gonna fill up and thats that.