Look at it this way. The only practice that is truly effortless is one without technique. Any technique is effort, even if subtle to the point of being hardly noticeable.

TM, mantra, mindfulness, candle gazing, etc... are all single-pointed focus techniques meant to focus and concentrate mind. Focus is effort.

Do nothing meditation, also called just sitting, resting in awareness and choiceless awareness dispense with technique and hence are effortless practice. It's just being and in fact that is your natural state even though it's not realized.

Effortless practice helps one realize the distinction between direct experience and thinking about direct experience. It's a very subtle but immensely profound difference.