Central mode should require the cyclic go back to neutral before accepting new input. But in the Ka-50 the instant you hit the trim button the helicopter starts piling on whatever input is currently receiving.
To test - ensure 'Central' mode is enabled in Ka-50 special options, start from ground or hover, pitch forward to increase speed, hit trim button at desired attitude - the aircraft immediately pitches even further forward.
In comparison the Mi-8's trim is behaving as advertised.
The Ka-50's 'central' trimmer mode seems to be an inadvertent copy of its 'default' option.