The eccentric part of a lift is the part where the muscle being worked is becoming longer; for example extending the arm during a bicep curl or lowering the bar during a bench press. It has been shown that this part of the lift can tolerate more load (up to 30% more!) and more reps before failure.
It can therefore be advantageous to continue training the eccentric part of a movement after failure is reached in the concentric part (opposite of eccentric).
Alternatively you can have your spotter add resistance when you being the eccentric portion of your lift (by adding weight or pressing on the bar). Eccentric training uses less energy spent, but still causes more muscle damage (and muscle soreness!) which leads to more muscle built.
Keep in mind that this also means you will need more time for recovery so we would speculatively recommend using the added eccentric load method one a week.
Doing eccentric training at a load over 100% of your concentric 1RM has been shown to give more strength gains (but not necessarily more hypertrophy) compared to lower loads or no eccentric portion. It is therefore important to never skip the eccentric phase of a lift, even if you are not adding the load! Control the weight, allowing for a longer time under tension and thus more strength gains!
Sources:
1. English KL et al. Early-phase musculoskeletal adaptations to different levels of eccentric resistance after 8 weeks of lower body training. Eur J Appl Physiol. 2014 Nov;114(11):2263-80.
2: Roig M et al. The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis. Br J Sports Med. 2009 Aug;43(8):556-68.
