Lead-in wire connecting radio to antenna appear capacitive due to its intrinsic shielding. The trimmer adjustment in the radio chassis is a variable capacitor and the triming process set the absolute capacitance of your particular antenna + lead-in wire (they vary from unit to unit) dead nuts on to the radio's design center.
Others have given you the proceedure which is basically 'peaking' signal strength using an existing weak signal at the bottom of the AM band and should also be documented in the AIM as well as the Delco Radio Service Manual. The net effect of 't*******' the antenna is actually quite marginal. Expect to see a measurable performance difference under 'fringe' reception conditions and no observable difference while listening to stations in a healthy/robust major metropolitan area.
Others have given you the proceedure which is basically 'peaking' signal strength using an existing weak signal at the bottom of the AM band and should also be documented in the AIM as well as the Delco Radio Service Manual. The net effect of 't*******' the antenna is actually quite marginal. Expect to see a measurable performance difference under 'fringe' reception conditions and no observable difference while listening to stations in a healthy/robust major metropolitan area.