The key element to Robin Trower sound is the use of the uni-vibe pedal. This is the Hendrix connection that makes the comparison between the two so common, and as I said, Trower was clearly influenced by Jimi although they were contemporaries.
To quote the all-knowing Wikipedia: "The Uni-Vibe (or UniVibe) is a foot pedal-operated phaser or phase shifter for creating chorus and vibrato simulations for electric organ or guitar. It was introduced in the 1960s by Shin-ei, and was intended to emulate the "Doppler sound" of a Leslie speaker. Though not a very successful Leslie simulator, the Uni-Vibe has become an effect in its own right, putting its stamp on tracks like Robin Trower's "Bridge of Sighs", Jimi Hendrix's "Machine Gun" and Pink Floyd's "Breathe." The effect, though often associated with chorus, is in fact created through a staggered series of phasing filters, unlike the usually aligned filters of a normal phasing effect."
To sum that up, if you like Jimi, Floyd, and Trower, you need a uni-vibe. A word of warning—they are very addictive and I'm in a 12 step program ween me off of mine.