I've always trained with a partner (the same one for years) so this was something I couldn’t wait to try out. We tried it last night we worked back/arms. I had my partner push on the negative while doing rows, curls, and tri extensions. It was fun listening to him grunt and huff while trying to push against me doing the rows... He's pretty stout and on one of the hammer row machines I had to put my full bodyweight on it to control his negative. Reps per set went down CONSIDERABLY! It'll wear you down quick, especially using your normal working weight. Pump was almost instant.
A great change up, something I've needed for a while. I woke up this morning sore as hell, a feeling I haven't had in a while and I DO BELIEVE that if you wake up sore it means you've done something right the day before as long as it’s the proper sore not anything out of the ordinary of course.
I suggest that you try this on machines before you attempt it on free weights, you may be surprised on how much force you have to apply to your partners negative. With a machine it’ll be a bit safer.