For me, the negative reps (eccentric) are more important than forced. Try those. have your spotter get you up completely (full assistance, to a near lock out position) and then as slowly as possible, lower the bar. Try to count to 10 (or close to it) before the bar touches your chest. Your spotter will then get you back up top, repeat. I go for 2-3 reps at the end of my set. Just a word of advice...if you are natural or take a long time to recover, be careful with these. The fascia needs to heal, as well as the CNS. 2-3 times a month per muscle group work for me.
I always feel like im injuring myself when i force reps, because my form is shot.