Lately I've been seeing more and more people in my gym doing this. They will be training with partners and instead of using weights, the partner will provide the resistence. For example, one guy was doing lateral raises as his partner tried to hold his arms down. Or one guy will hold the ends of a towel behind the other guys head while he holds the middle and does tricep extensions. I thought it was stupid when i first saw it, now more and more people are doing this. just seems like an inefficient substitute for machines. Anyone heard/done it?