Here’s how I work with the hip flexors. While I show some variations for clients who prefer more direct contact, my usual approach for iliacus and psoas is one of broad contact and slow sinking.
First, these muscles are rarely touched, and so it doesn’t take a lot of stimulus for them to reduce their tone and sensitivity. I find that simply making contact and waiting can produce fairly pronounced softening in a short period of time. It’s like they’re waiting for a chance to take a break.
Second, these can be sensitive areas, and it’s worth keeping the human in mind as you approach them. Attempts to dig into psoas can be anxiety provoking for some people, and even the best work can be counteracted if the contact is perceived as threatening. Why not start with what’s friendliest to the client’s nervous system and see what results you get?