How to use FF_CONSTANT for Force feedback for Linux?

Question

I am not able to utilize FF_CONSTANT force effect. My try code is:

struct ff_effect joy_effect_, joy_effect_2;
if (iwantconstantforce)
    {
      joy_effect_.id = -1;
      joy_effect_.type = FF_CONSTANT;
      joy_effect_.direction = 0x0000;  // down
      joy_effect_.replay.length = 100;
      joy_effect_.replay.delay = 0;

      joy_effect_.trigger.button = 0;
      joy_effect_.trigger.interval = 100;

      joy_effect_.u.constant.level = 65535;
      joy_effect_.u.constant.envelope.attack_length = joy_effect_.replay.length / 10;
      joy_effect_.u.constant.envelope.fade_length = joy_effect_.replay.length / 10;

      joy_effect_.u.constant.envelope.attack_level = joy_effect_.u.constant.level / 10;
      joy_effect_.u.constant.envelope.fade_level = joy_effect_.u.constant.level / 10;
    }

I am able to produce FF_SPRING and FF_DAMPER effects with following codes.

if (youwantdampereffect)
    {
      joy_effect_.id = -1;
      joy_effect_.direction = 0;  // down
      joy_effect_.type = FF_DAMPER;
      joy_effect_.replay.length = 20;
      joy_effect_.replay.delay = 0;
      
      joy_effect_.u.condition[0].right_saturation = 65535;
      joy_effect_.u.condition[0].left_saturation = 65535;
      joy_effect_.u.condition[0].right_coeff = 65535 / 2;
      joy_effect_.u.condition[0].left_coeff = 65535 / 2;
      joy_effect_.u.condition[0].deadband = 0;
      joy_effect_.u.condition[0].center = 0;

      int ret = ioctl(ff_fd_, EVIOCSFF, &joy_effect_);        // upload the effect
    }

    if (youwantspringeffect)
    {
      joy_effect_2.id = -1;
      joy_effect_2.direction = 0;  // down
      joy_effect_2.type = FF_SPRING;
      joy_effect_2.replay.length = 20;
      joy_effect_2.replay.delay = 0;

      joy_effect_2.u.condition[0].right_saturation = 65535 / 2;
      joy_effect_2.u.condition[0].left_saturation = 65535 / 2;
      joy_effect_2.u.condition[0].right_coeff = 32767;
      joy_effect_2.u.condition[0].left_coeff = 32767;
      joy_effect_2.u.condition[0].deadband = 0;
      joy_effect_2.u.condition[0].center = 0;

      int ret = ioctl(ff_fd_, EVIOCSFF, &joy_effect_2);        // upload the effect
    }

I do not find any info about what is constant force effect feels like or when it makes sense to use it. Can somebody brief its importance and usage?

Thanks :)