Push Hands (推手) is a well-known basic training exercise of Taijiquan. Partners stand facing each other and each attempts to unbalance the other.
In the ZMQ37 style as taught by Ben Lo, the emphasis in push hands training is not on elaborate formal patterns, nor on particular techniques that will give an advantage. The emphasis is entirely on relaxation. We gradually learn to control both our own and our partner's tension, ego, and fear. This emphasis on relaxation naturally yields all the attributes required for push hands training and self-defense: sensitivity, neutralization, and instant natural "rebound" power that emerges spontaneously as required by a situation.
No fancy technique or special movement pattern matters at all. There is no if-A-then-B. Relaxation is the only key.
The most common error in push hands training is using too much strength, failure to relax.